I've played with iPad 2. Everything was just awesome and the game unbelievable beautiful (compared with iPhone 4 and other smaller devices)
In addition it is of course easier to press skills, but your character looks like a boss within great resolution (and bigger screen)

Now i am playing with iPad 3. Seriously...its damm epic. Everything looks better, but if you want to save some bucks, you can also buy an iPad 2 or acheaper android devices with bigger screen.