I have SRAM Red on my road bike, Shimano Ultegra 6600 on my touring bike. I also have two mountain bikes with SRAM components. There's a lot to like about both brands, but there are distinct differences between them:
- SRAM components are usually lighter than Shimano components of the same cost.
- Shimano components are quieter and smoother than comparable SRAM components
- Both brands shift well. Lots of people are freaked out by the idea of DoubleTap, but in practice it's really very similar to Shimano. You make the exact same movements to shift (big push = bigger ring, small push = smaller ring), you just do it with one lever for SRAM versus two for Shimano. I swap back and forth between SRAM and Shimano regularly and have never missed a shift
- Shifts with a SRAM rear derailleur are very fast and very precise. Shimano, feels a bit lethargic and mushy in comparison. Rear shifting is what sold me on SRAM to begin with...
- SRAM levers fit my hands much better than Ultegra 6600. Ultegra 6700 and Dura Ace 7900 levers aren't as bulbous and bulky as 6600 and 7800, but they're still not quite as comfortable for me as SRAM.
- SRAM brake/shift levers offer built-in reach adjustment. If you have smaller hands, like I do, this is a very nice feature
- SRAM doesn't offer triple chainring shifters or cranks. The Apex and Rival groups for 2011 do, however, feature a mid-cage rear derailleur that will handle an 11-32 cassette, which will give you the same gearing range as Shimano's road triples (52/39/30) with a 12-27 cassette
Ultimately, you should buy SRAM if you value performance (light weight, quick shifts) above all else. Buy Shimano if you want smooth, quiet shifting. I like them both, but would give the nod to SRAM. The ergonomics work better for me, I love the lightening-fast shifts, and the noise doesn't bother me.