When I was researching my components for my 'best' bike some years ago, SRAM were new in the road bike market and I enquired of a well-respected member of CC (a top mechanic with much experience and a good guy) about the differences. His response was along the lines of:
"In their shifters, Campag have about 23 moving parts**; Shimano have about 35; SRAM have about 16. That makes a big difference to possibility/ease of servicing and is worth considering. I like the SRAM stuff."
I got SRAM Rival, which was the bottom end then and loved it from the first. I've ridden Shimano a few times and never settled with the brake/ shifter thing. Double-tap always seemed more logical/easier to me.
On the EdFoC Night Ride t'other year, we had someone whose Shimano 105 cable broke at the brifter. Not something the travelling mechanic would attempt to fix out on the road, so the lucky rider got to do the remaining 70km on a 2-speed bike ! He made it, though ...
**all numbers are for comparison only - they are not intended to be accurate, as I haven't counted them all ...