Re: Con v Dex
AC is actually pretty hard for a wizard to raise, unless he spends magical resources (and rounds of spellcasting) to improve it. I always thought that if a wizard of mine was even a target of an attack, I wasn't trying hard enough. At low levels, use cover, concealment, non-wizardly clothes, and the presence of the fighter to make yourself an unlikely target. A 1st level wizard isn't much in combat anyways, so your party won't lose much here. At higher levels, there are plenty of spells that block the enemy's line of sight, some of which don't touch yours, so use those to prevent direct attack (Leomund's Tiny Hut, Major Image, wall spells). At that point a higher AC becomes absolutely useless for you. Every now and again an enemy will slip past those defenses, and when they do, you'll probably want more HP to soak a blow and allow yourself to escape than reduce your chance of being hit from 95% to 80%.
Re: Human v Gnome
You lose a feat and one skill point per level; in return you gain 1 hp per level, +1 AC, +1 attack, some nifty SLAs, and a large nose. I could really go either way depending upon the concept. Frankly I'd call this a wash.
Re: Con v Int
You get a +3 on stuff if you choose Con; you get a +1 to things if you choose Int. The skill point and save DC is nice, but raising your Con ever after will be a damn nuisance, while raising your Int from 15 isn't that bad a starting place.
Re: Specialization
If you want a cheap extra spell per day, be a Diviner. You'll generally be able to use one Divination spell in your repitoire per day, and you'll have the rest of your spells for more combat oriented stuff. If you want to do some serious focusing, specialize and take Focused Specialist from the PHBII; you'll get 3 extra spells for your speciality, something that the usually spell-starved wizard can redialy use. Yes, you give up 3 schools of magic for this, but you'll just have to let the group know that you're not a catch-all versatility wizard, you're a gum-just-ran-out but-kicking wizard.