SRAM – (+) Brakes are waaay stronger than Shimano, shifting stays in line better too
(-) Feels a little like an Italian group, IE agricultural and makes more noise
Shimano – (+) Superslick shifting, easy to get spares newer groups now have hidden cable's, cranks are the stiffest out there.
(-) Not as nice to look at (IMO), brakes can be poor and soggy.
Both setups are excellent and as it has been said before see what you can get on a deal. I prefer SRAM but would gladly ride Shimano (and have done,lots)