Well since this thread is over a month old, and the OP likely has made a decision, I'll throw in my 2 cents for anybody else thinking about this. I think the Wii U is far too early in its life to be a must own, and even though I bought a Vita, and don't regret it one bit, I wouldn't be devastated if I didn't own it either. However, I just recently decided to pick up the new iPod Touch 5th Gen and I must say that if you want to spend $300 right now, and don't have an iPad 2 or higher or an iPhone/iPod 4 or higher, then seriously consider this. It took me very little time to get used to the controls of not having buttons and there are literally dozens and dozens of great, addicting games, and many of them are either free or go on sale for .99 or free all the time. For those of you that say you can't get the same experience on an iPod/iPhone/iPad that you can on a dedicated gaming device, I agree. However, if you can find yourself playing a game for 2-3 hours at a time, (GTA 3, GTA Vice City, GTA Chinatown, Lego Harry Potter, Infinity Blade, Bastion, The Walking Dead, Mass Effect Infiltraitor, Need for Speed "Take your pick", Skylanders, Final Fantasy 1-4) and only ever spend between $0-2.99 per game, then I say you've got yourself a truely great device. The 5th Gen Touch has 32gb of space and that's a lot. It has the bigger screen and many games are updating for it and almost all new games are optimized for it, plus it has Siri which is always fun to screw around with.
Knowing everything I know about the devices, if I wanted to game, had ~$300 to spend and already had either a PS3 or a 360, the next thing I would be buying would be either iPod Touch 5th Gen or an iPad Mini.