Chris, checkout reviews such as that at Photozone or Lenstip...
http://www.photozone.de/canon_eos_ff/752-canon_40_28_ff
IMO the 40mm better of the two. I have the 50 II and it was a bargain but always wish I'd got the 40. I have to stop down the 50 to f/4 before it starts cleaning up, coma is a problem before that - and I'm using a crop chip!
The 40mm as tested above seems to put a good foot forward on a full frame chip even at f/2.8.
There are others in that focal range, but many are significantly more expensive, and even then some still need to be stopped down to behave themselves.