I honestly think they are all much of a muchness. Comparing the top of the line components from all 3 companies....if one brand was truly any better than the rest, then ALL of the pro mechanics, along with ALL bike manufacturers, would insist on using that brand on their bikes. As it is, there is a pretty even spread across the market.
So to answer your question, I don't think it matters either way. For the record, I have Ultegra on the roadie, and SRAM Red on the TT bike. The Red provides slightly sharper gear changes, but thats to be expected.
IMHO there is no best.....I love Campagnolo Super Record but I don't rate it as "better" than Shimano or SRAM. You find what you like and you stick with it until you find something better.
I changed from tiagra to ultegra, huge difference, brakes are better, shifting is much snappier and responsive.
Shimano's Ultegra 6700 received high marks for its brakes, front shift performance, light weight, new lever shape and hidden shift cables from Cycling News. The group-set missed the mark on several function designs, such as the long shift lever throws and the obstructing brake cable cover plates and screws. Cycling News commended SRAM's Force on its braking power and ergonomic lever feel, as well as its short lever throws
I think go with the one that has the hoods that fit your hands the best. Also the one that allows you to operate the shift lever the best. I find i twist too much using shimano hence why i have been using campy since the early 90s.
I have used SRAM Rival and Shimano 6700 Ultegra. I think that the shifting is a bit smoother on the Shimano but I prefer the double tap action of the SRAM. I also think that the front derailleur on the SRAM is better even though its changes are a bit more clunky. I think it gives better clearance to the chain which makes it easier to set up and gives you a larger usable range of the rear cassette (or maybe the set up on my Ultegra is crap...)
At the end of the day i would be happy with either. The SRAM is lighter and I think a bit cheaper than the Shimano. If the frame was BB30 I would go with the SRAM if not then I would go with which ever one I could get the best price on.
I had the choice between Ultegra and Force on my bike - decided on Force for a few reasons - and not vey scientific.
I don't like the look of the Ultegra Cranks (dunno why, just don't)
I liked the idea that Force is basically the same as Red except for weight and materials used
SRAM is not as common - I like to be different
Double Tap appealed to me as a new cyclist
That all been said, one of the bike experts I spoke with to get advice on such things made a great comment - once you are in gear - it doesn't matter what brand or level of gear train you are using ......
as long as the road is empty, flat and straight, with you at comfortable cadence. Change any variable and ......