Have a read of my blog post on this very subject!!
Real Racing HD vs Need for Speed Shift hatenintendo blog
It depends what you want but GT Racing: Motor Academy has also just released so worth you looking at that too. I think GTR will be my most played racer on the iPad now.
Like I say it depends what you want:-
RR = hardest to be super fast on. I think it runs TOO fast. You can go through the bends way too fast even with the hot hatches class, so it's a bit crazy. Online leagues but not racing onscreen together.
NFS = more linear career progression. Car upgrades like Nitro, so it's more arcadey. Only single player. More accurate car speeds IMO.
GTR = Loads of content, more like Gran Turismo in it's setup i.e. different events you can do in a less strict order. Car upgrades/tuning as well. Great selection of real cars that drive different to each other (the Caterham is crazy hard to control!). Nicer framerate then NFS (but not as fast as RR!). Good customisable control schemes. Proper online racing for 6 cars I think.
Overall it's my favourite but they are all good really. Get em all!!!
I will do a blog review on GTR soon... hope the other one helps?