Get Real Racing because you'll be waiting a long time for NFS: Shift. Real Racing is also an amazing game, one of the best on the iPhone.
Real Racing...and get it right now! Shift isn't going to be out for a while. Real Racing is the best racing game on the iPhone and Firemint are awesome devs who have made some awesome updates to the game recently, showing their commitment to the game unlike EA who have done no updates on Need for Speed so far. Still haven't made fixes for 3.0 yet... (poor!) Wait for Shift to come out and there will be more than enough early reviews to help you make up you mind before purchasing.
If you can wait, wait for NFS, but it'll still probably a few months from now. If you can't, then get Real Racing.
Depends on what you want. Do you want SIM or ARCADE play? RR is $9.99 now and NFS Undercover is $6.99 (cheaper and should hold you over until Shift arrives if you like arcade racers). For pure SIM racing RR is it. Nothing else matches it on the iPhone. Shift isn't suppose to be released until sometime this September I think, so if you wait you are looking at a long wait.
It's hard to judge now, there's barely any info on the iPhone version of NFS Shift. Real Racing is still pretty good though. Though it's only fair to judge when they both come out.