I'm inclined to think that that link is an error, and it's supposed to link to the Whirlwind ability. Here's my thought processes on the subject.
First, it's also a supernatural ability. That scans better with the sorcerer ability. If the sorcerer ability was mimicking a spell, it'd likely be a spell like ability.
Second, the monster ability actually turns you into a whirlwind. The spell doesn't. The effect is similar (y'know, 'cause there's a whirlwind and all), but in terms of applicability they're pretty different.
Third, the monster ability actually has rules you can for existing as a whirlwind. Things like how YOU function, rather than how things are affected by you.
So I'd say it's faiiiirly likely that the link is in error. Can't be certain, but it does seem likely.
Now, let's go over all your questions and see if we can help out.
Is this any different from the Whirlwind that you would turn into if you cast Elemental Body (air elemental) and then used the whirlwind ability?
Assuming that it really is the Monster whirlwind and not the Spell whirlwind, and that you're staying the same size, yeah, it would be. With the exception that you don't gain the bonuses that Elemental Body would give you.
Also, this power says that you become a 10-foot tall whirlwind... what if you are NOT medium at the time you use the ability? Does a small character using it still become a 10 foot whirlwind? Does a large creature?
Per RAW, you would still turn into a 10-foot whirlwind. It would definitely not be an unreasonable house rule to allow you to be enlarged when you change, but that would be a houserule, so you'll have to ask your DM.
Also, as a general rule about whirlwind, not just related to this Djinni bloodline power... would a 10 foot tall, 5 foot wide whirlwind created by a medium creature turning into one still be considered a "medium" whirlwind that could then only damage and lift small creatures? Or, since it is 10 feet tall, would it be considered "large" and then be able to damage and lift medium creatures as well, even though it is only 5 feet wide and not 20 like a typical large creature?
According to the whirlwind text
The whirlwind is always 5 feet wide at its base, but its height and width at the top vary from creature to creature (minimum 10 feet high). A whirlwind's width at its peak is always equal to half of its height. The creature controls the exact height, but it must be at least 10 feet high.
The whirlwind form does not provoke attacks of opportunity, even if the creature enters the space another creature occupies. Another creature might be caught in the whirlwind if it touches or enters the whirlwind, or if the whirlwind moves into or through a creature's space. A creature in whirlwind form cannot make its normal attacks and does not threaten the area around it.
Creatures one or more size categories smaller than the whirlwind might take damage when caught in the whirlwind (generally damage equal to the monster's slam attack for a creature of its size) and may be lifted into the air. An affected creature must succeed on a Reflex save (DC 10 + half monster's HD + the monster's Strength modifier) when it comes into contact with the whirlwind or take damage as if it were hit by the whirlwind creature's slam attack. It must also succeed on a second Reflex save or be picked up bodily and held suspended in the powerful winds, automatically taking the indicated damage each round. A creature that can fly is allowed a Reflex save each round to escape the whirlwind. The creature still takes damage but can leave if the save is successful.
Creatures trapped in the whirlwind cannot move except to go where the whirlwind carries them or to escape the whirlwind. Trapped creatures can otherwise act normally, but must succeed on a concentration check (DC 15 + spell level) to cast a spell. Creatures caught in the whirlwind take a –4 penalty to Dexterity and a –2 penalty on attack rolls. The whirlwind can have only as many creatures trapped inside at one time as will fit inside the whirlwind's volume. The whirlwind can eject any carried creatures whenever it wishes as a free action, depositing them in its space.
If the whirlwind's base touches the ground, it creates a swirling cloud of debris. This cloud is centered on the creature and has a diameter equal to half the whirlwind's height. The cloud obscures all vision, including darkvision, beyond 5 feet. Creatures 5 feet away have concealment, while those farther away have total concealment. Those caught in the cloud of debris must succeed on a concentration check (DC 15 + spell level) to cast a spell.
Format: whirlwind (3/day, 10–30 ft. high, 1d6+6 damage, DC 15); Location: Special Attacks.
Given your answers to the above, I wonder on people's takes on if this Djinni power is gimped, the same, or better than the comparative of using elemental body first and then transforming into a whirlwind?
In my opinion, it's neither better nor worse. It's different. Keep in mind that it's free. It's not DESIGNED to necessarily be better or worse than the spell. It's designed to be a neat thematic effect with with some cool bonuses. You get it for free (sort of . . . there is an opportunity cost, but it doesn't take away from your spells per day, at least), and while it is weaker in some aspects than the spell, it's stronger in some others.
Here's a few things to consider.
It's a supernatural ability. You can activate it when grappled and get the heck out. This is worth. A. LOT. My entire party almost got TPKed a few sessions ago when an enemy caster Super Black Tentacled the entire group and no one could get out because our melee has no options and our spellcasters couldn't spell-cast. Hooray for clerics with the travel domain, heh. But, yeah, supernatural ability. It's worth more than it seems at first glance. It's ALWAYS good to have options, and the more options you have, the better. The more different options you have, the MORE better. :)
It also seems to keep your size. At ninth level you're casting the rough equivalent to a 5th level spell. Not terrible, I guess.
While it's not explicitly stated, it's sorta hard to be humanoid and a whirlwind. You should clear this with your DM so he/she knows your intent to use the ability defensively and isn't surprised when you do it (and thereby wants to quash the ability), but if he allows it to make you non-humanoid, it would, be extension, make you an elemental. Elementals are immune to all sorts of fun stuff, like criticals, poison, drowning, suffocation, et cetera. You could probably even clear a room of harmful gases, possibly even conjuration spell ones. Your mileage may vary, and stuff. One of my favorite oracle abilities is the Energy Body ability for the Life Oracle. As for healing, it's terrible, but as a defensive ability, it's superb. Posssssssibly not how it was intended, heh, but it works anyway. ;-)
Sorry for the long post with so many questions, but I have a player who wants to take this power and I'm just not familiar with whirlwind, and them more I read other posts and the rulebook, the more confused I get.
Woops. That means you're the DM. Guess it's obvious I respond in segments rather than reading the whole post first. xD Anything I said up there that was "talk to your DM," change that instead to, "Using the facts that have been presented to you, make a really awesome DM-decision that enhances your gameplay for you, your player, and your group."
Personally, I say, if he's doing it because he thinks it's a cool theme, give him what he wants. If he's a powergaming little munchkin, do anything but. ;-)
Forgot to ask... does being a whirlwind (though it doesn't say it in the rules) make you also immune to non-magical weapon damage? I guess from a RAW standpoint I see no reason for that, and maybe that is unbalanced... but how would swinging a sword or shooting an arrow at a whirlwind damage it in any way?
Hmmmm. Maybe I really SHOULD start reading the whole post first. >.>
As I said above, this ability isn't well defined. It's going to be up to you to decide exactly how this ability works in relation to What Type of Creature the character becomes, How His Type Interacts with environment and other characters, and What In-Game Benefits you want him to gain from these changes and interactions. There are two ways you want to look at this: Balance and Logic. From a balance perspective, is it imbalancing or too powerful to allow him to actually become an elemental when he uses the ability? Is it too illogical not to? Those are the questions you'll have to consider and decide upon.
As to your specific question, elementals are not immune to non-magical weapon damage. They're not immune to damage at all, unless their statblock says so (generally that'd be their element). What you're thinking of is "How the heck does someone hurt a ball of air/fire/water??" The answer, in game terms, is, "By overcoming its damage reduction." Most elements have a fair bit of DR/-, so they're ignoring a fair bit of damage. That's the balance and logic components working in tandem. Neither one is perfect, but they meet a fair compromise that allows us to play the game without getting bogged down by inanities.
Yet ANOTHER thing I forgot to ask... since the rules for whirlwind form state that this form does not provoke attacks of opportunity, but it also seems that you are able to cast spells in this form provided you can get around the component restrictions (eschew materials, still spell, silent spell, etc), do you provoke attacks of opportunity when casting a spell in this form?
Hmmm. I actually read that line as "does not provoke attacks of opportunity due to movement," but that isn't explicitly stated. Another question you should be asking yourself is, "Is the sorcerer even allowed to cast spells?" He lacks a voice and fingers. How is he casting? I think the Whirlwind description reads the way it does, specifically addressing movement-based attacks of opportunity the way it does, because that's the only type of attack of opportunity you can actually provoke in this form. That's a severe stretch of logic based upon very flimsy syntactical observations, but it does exist. Sadly, you're probably not going to find a really hard ruling on this, so you may have to decide this one on your own. Again, ask yourself if the sorcerer gains too much by being able to talk, cast, act, whatever in this form. But honestly, I would say that he probably doesn't get to cast. There is precedent for air elementals being allowed to speak, since they can control air and thus "create" a voice, but he definitely doesn't have fingers, heh.
Anyway, hope this helps. Don't be afraid to ask more. Hopefully if you have fewer questions people won't be so scared to tackle the post before you get to the bottom of the page. ;-)
Edit: Also, check your private messages.
