Well, in order to maximize the summoning spells you're going to have to take the Augment Summoning feat. It's just too good to pass up. I'm also playing a Sorcerer with the Varisian Tattoo trait (Campaign Setting or Rise of the Runelords) in order to get the +1 level bump to Conjuration spells. Makes the duration longer before you can get Extend Spell as a bonus feat at level 7 with the Celestial bloodline.
You get a similar effect specializing in Conjuration as a Wizard, and using the [Summoner's Call] to add 1/2 your Wizard level to the duration.
The problem with only summoning elementals is that 90% of the bad guys in the game are going to have the "Evil" alignment, and if you're Good, then all your summoned animals get the Smite Evil ability added to them. Nothing like hitting that goblin with a Smite Evil from the Augmented Celestial Riding Dog at 3rd level. The other players just stand back and watch the carnage. It gets better with Summon Monster III and wolverines' "attack, Rage, and watch the ripping to shreds" tactic. When in doubt cast your highest Summon Monster for 1D4+1 of the lower types, and flood the battlefield with low-hit wonders until you've swarmed the bad guy under them.
On the other hand, with elementals you never have to worry about undeads' paralyzing touch or devils' Fear abilities. An elemental is a construct - it ignores such attacks. Elemental Speech is even a spell now, so you don't have to put 4 skill ranks into Linguistics in order to communicate with your elementals.
I would say that Summon Monster has more side abilities and special attacks, while elementals have more direct attacks and immunities.