Ipad has a bigger screen and is overall nicer to play most of time (and with more impressive graphics). However, there are some kind of games that work better on iphone, in my opinion, like endless runners. Is there any kind of game (or any particular title) that you prefer to play on your phone over tablet?
There are some games like Threes and Impossible Road which don't particularly game or lose anything either way. One game I prefer on the iPhone is Goblin Sword - but that's just because the controls work better on an iPhone.
Games i like better on the phone than the ipad: Threes Tiny Goalie Doodle Jump Batman Monsters Ate My Condo / Super MAMC Flapthulhu Dungelot
I prefer first and third person shooters on the iPhone, just because they are easier to control on the smaller screen. I also like racers on the iPhone because tilt controls on the iPad are just too unwieldy for me.