In reply to Philip:
Totally depends what you normally shoot. Nifty fifty will give you a mild tele. 35 or so equals a 50 on full frame and so forth. If you're not sure I'd really try to borrow both 35 and 50 and see which one you like best.
I have a 15mm, a 35mm and a 77mm prime (I also own a couple of zooms but I mostly use the primes these days) and I use them all in bursts, depending on how I feel and what I'm shooting. Some days are mostly wide angle days, some days I just like isolating details with the 77 and some days just feel like 35 days.
Of course another factor is that if your camera can use them, you can pick up oldy but goody 50s for cheap.
Good luck
Paul