SRAM: I know nothing about it because I have never used it.

Shimano: have used 105 and Ultegra. Both fine. If keeping witb Shimano I would stay with Ultegra because it is a good groupo. Shimano is very smooth.

CAMPY: I LOVE my Campy. I have a full Campy Chorus drivetrain with a carbon 53/39 crank. I like the hoods -- not the big "ditch" as in Shimano, giving me more confidence and better reach/grip. The shifting is wonderful -- can drop from lowest to highest in one quick shift (great when going down hill after a steep climb!) or go from high gear to low gear, jumping to multiple gears in with just one shift (great when climbing or need to increase cadence real quick). I don't think it is as smooth as Shimano, it is more crisp and quick. As for maintenance, if Campy breaks, you can fix it. Shimano needs to replace. Don't know about Sram. One more thing, when putting the cables on your bike, Shimano is very visible at the front of bike, needing only one groove on your handlebars for one cable to wrap under tape (all else visible). Campy is "hidden", using two grooves on handlebar, but if bar only has one groove you can make it work, wrapping cables under tape and making a cleaner appearance and keeping shifts crisper. Sram needs 2 grooves on handlebars and you have to have a 2 groove handlebar to wrap cables -- 1 groove won't work.