In general consensus would be that crossblooded is the right choice for most optimized sorcerers. But then again most optimized sorcerers know what they want to be doing with their spells known.
The best guide to bloodlines, in my opinion, would be here. It outlines a lot of your powerful options and gives you the roles of each.
I'll alert you now that there are two detractors to the awesomeness of the Arcane Bloodline. The first is that Crossblooded can very effectively weave between bloodlines making for a mix and match bloodline that can equal Arcane. The second is that since Familiar Folio most bloodlines can grant a familiar.
You can of course, go for broke and nab both a bonded item and a familiar through crossblooded with Arcane. More spells is always nice.
The Shadow Bloodline alone though is pretty amazing. In my eyes this is one of the most fun bloodlines. As a Fetchling you can trade out Skilled for Shadow Magic which definitely encourages a Shadow Bloodline. There are even three ways to ditch the lackluster ability, Shadowstrike. Thanks to Familiar Folio you can trade out it out for a familiar with a cool version of Cause Fear. There is also a Shadow Bloodline exclusive archetype that can't combo with Crossblooded within Blood of Shadows. Finally, there is the Umbral Wildblooded which can be more useful and again doesn't combo with Crossblooded.
I like Arcane Bloodline more for Eldritch Heritage. Umbral Wildblood has a nice Arcana, but I'm happy with a familiar that has a small bonus especially if you can get it with the School Familiar (Illusion) archetype. What it comes down to is either the archetype from Blood of Shadows or Crossblooded and being mostly a pure Shadow with cherry picked abilities from another.
Personal opinion... Crossblooded. Especially for Psychic, Fey, Serpentine, or Rakshasa. You can't go wrong with that answer.
However I don't really get the formula of Psychic, Fey, Serpentine, or Rakshasa.
I read on them a bit and Psychic has a good level 1 spell that trumps Shadowstrike but the rest I can't see it.
Is it for the bonus they are better or something else?
If you can explain a bit it would be grateful.
No problem!
What it comes down to for most is Arcana. You don't really need a better 1st level ability since the familiar you can get in place of Shadowstrike is aces. Actually, as a Fetchling you might consider trading out, Nighteye, instead either for a nice 3rd or 1st level ability.
The Psychic Bloodline changes your casting to being psychic in nature, which is a matter of preference most people are still making stary eyes at. You should look it up to see if it suits you. Otherwise you can get Undercasting Prodigy over Shadow Well which goes a very long way to solving the problem andreww mentioned. You can have around two to three spells known every level past 10th this way. (Beating out the non-crossblooded Shadow bloodline. :p ) The Thoughsense is also pretty nice, but you have to pick between that and either Shadow Well.
Fey is, again, mostly for the Arcana. It's a great boost to DCs and comes with a really nice and complementary list of spells. It's a highly specialized choice.
Serpentine, again, that arcana. The familiar is funny enough trash because you have better familiar access. Some cool spells.
Rakshasa is the only one the arcana sucks for. What doesn't suck are the abilities and spells. Seriously, this is a fantatic list of options such as Mind Reader than can make for a very strong character.
This is categorically untrue. Crossblooded is an absolute trap for the majority of sorcerers. It is mostly used by other classes who want to deal direct damage and grab +2 damage/dice with a single level dip for orc/draconic.
I was trying to say if you are going for high optimization, regardless of if that is blasting or enchantment or whatever, it is ideal. In other words, specialised builds. You only end up marginally behind a few times in your career for being ahead the rest of the time. I make this statement partly off the back of the guides for sorcerers I have read.
There's actually some trickery to get around this problem. My favorite being Veiled Sorcerer. It's fun for Words of Power too. Another is Undercasting Prodigy. True, you are at level 10 before it is no long a problem, but for the first few levels you feel the sting less only to be hurt at 4th, 6th, and 8th. Will I take 3 bad levels (one if you play Veiled Sorcerer and none for WoP) in trade for a stronger character the rest of the time? Yea, pretty much.
But again, you should have a very firm idea of what you are doing to attempt it.
Sorcerers live and die based on the number, range and versatility of their spell selection. This is also why the Human FCB (also available to half elves and half orcs) is so incredibly valuable.
Fetching can too as of Blood of Shadows. Look at any genie kin for reference to how.
Though, to be honest if number, range, and versatility are all that matter to you play a Wizard.
Crossblooded is a really bad choice, especially if you're the team's only arcane caster. Losing 1 spell per level from your spells known is a huge hit. Imagine being level 5 before you get your first 2nd level spell, or level 7 for your 3rd level spell.
Crossblooded is really only good for dipping. Using a race that gets to use a favored class bonus to get additional spells known doesn't make up for getting them late, but at least you have more than you would otherwise.
I think you're thinking that you get all of the bloodline powers as a crossblooded sorc. You don't. You get both of the arcana, then you get to pick and choose between the bloodline's powers, feats and bonus spells each level you gain one. So you might choose Arcane Bond, Metamagic Adept, Shadow Well, Enveloping Darkness and Arcane Apotheosis as bloodline powers, say, but if you did you wouldn't also get abilities from swapping out bloodline powers or get the bloodline powers I didn't name there.
If you're not going with the sage bloodline (& you can't if crossblooded) you aren't going to have the intelligence to make that possessed trait work well.