I agree with Jay, AP and AD are both viable, you have to consider your team comp though, if you're particularly wanting to play Ezreal though you can choose either. While his auto attacks suffer somewhat if you go AP, you should only really be last hitting minions anyway in mid (or anywhere for that matter unless you're pushing), AP Ez can do his full combo for a heap of damage, personally I find AP to be suited to a more aggressive play style, you E in, then W > Q, most people back off, then you can follow up with an ult if you can get a kill.
Personally I don't find mana an issue, some people do but an early Tears will sort you out no problem. Yes some AP champs can go for more damaging options early, so this can leave you feeling a little weak at some points, but Ez can be a slippery sod. I tend to find playing as an AD carry harder, people want kills for carries, and with Ez these tend to rely on your lane partner setting it up, being too balsy will get you killed. If you can get a Soraka with you in bot though (woo for unlimited mana!), you can pretty much completely zone the enemy.
IMO both AD and AP are perfectly viable, and both can be fun to play, and that's the best part :)