I personally would just get an iPod for those few exclusive iOS games (of which there will steadily be fewer as Android continues to grow and overtake iOS in terms of sheer numbers).
There's a number of great games available on both platforms, though your options in terms of hardware are wider with Android. It's also much easier to tweak the phone, as jailbreaking simply isn't really something you have to deal with.
The Nexus S, which is available on Sprint, AT+T, and TMobile is an awesome phone that I can heartily recommend. Plus, it's $99 now, and it's among the few phones with pure, unskinned Android, and you can unlock the phone and flash different firmwares on it with basically ten seconds worth of effort. Also, in my experience, it plays basically every game smoothly, with the occasional hiccup during Dungeon Defenders.