Tastes great V Less Filling...
This is 1 of those things that Mountain bikers love to argue about.
However, I am SRAM. The shifters are just better in my opinion. Shimano is using their rapid fire system which uses the Thumb and index finger. I have used these and liked them until my current bike came with SRAM shifters. SRAM comes with uses the thumb only. It took about 10 miles (1 ride) for me to get used to the shifting and I never turned back. I am now a SRAM guy and will not look back.
In other areas, SRAM's Speed link system BLOWS AWAY Shimano's chain!
I am currently deciding on a new bike. A custom build that I am going to be picking out.
What I am thinking...
XX Crank-set
SRAM 970 chain and cassette
SRAM X.0 shifters, derailleurs.
I am not sure of brakes though. I use Juicy's now and I am happy, but Avid brakes squeal and squeal loud at times. I will likely stick with them unless I go with Hayes.
Pedal wise - Shimano PD (SPD) is the way to go. I use them and do not see myself changing.