I have Real Racing for iPhone (the old one) and NFS Shift. NFS Shift has dodgy car physics (like stupidly easy drifting) which ruins it a bit. Real Racing I think is a bit boring, with virtually identical cars, and just boring track racing the whole time.
The best racing game by a long way I've found so far is GT Racing. Over 100 cars, excellent car physics, some SERIOUSLY complicated and realistic car modding options (like adding weights to the front and back to improve handling) if you want to go that far. There are more fun races like a Model-T race, cool terrain like ice, tarmac, dirt, rain, and some seriously good graphics.
I don't know if GT Racing is available on the iPad yet, but if it is, I think you should get that. If not, go with Real Racing if you want something realistic, or NFS Shift if you want something "just for fun".