I've already posted that I've chosen SRAM and that I prefer the way it handles shifts to larger cogs. I've also gotten used to having a fixed brake lever that is not involved in the shifting action. That never seemed like a big deal when running Shimano but once I got used to having a fixed lever that didn't swing side to side just like the old days before integrated shifters I decided I liked it better, especially for hard braking situations like cyclocross.
I also like the way I can rapidly 'dump' cogs with SRAM in situations where I want to upshift a lot of gears quickly like topping out a climb onto a descent or sometimes during a sprint. With SRAM you can pull the shift paddle in towards your drops, and drop rapidly down through the cogs in a hurry. But mostly I like the way SRAM seems to stay in tune better and how it seems to give a solid one or two gear indexed action in both the upshift and downshift directions. My Dura Ace Shimano shifters required fairly frequent tuning tweaks to keep them operating smoothly and even after years of using Shimano I'd often find myself going up a cog and a half during shifts to larger cogs and then having to do a quick downshift to clean my shift. IME, SRAM stays in tune better and provides better indexing in both directions.
But I guarantee for every poster who tells you they prefer SRAM you'll get at least one that prefers Shimano and another who strongly prefers Campy. They're all good and they've all got folks who really like them so it comes down to your personal preference more than any of our opinions. There are some other considerations, Campy and SRAM integrated shifters can be rebuilt if you have trouble, Shimano is not designed for field rebuilds though some folks have figured out how to get them apart and back together again. Campy and SRAM shift without moving the main brake lever laterally which some like, some do not.
Really your best bet is to find a way to test ride bikes set up with each system. It doesn't have to be the exact same bike or the bike you intend to buy but see how the different systems work and see if you prefer one vs. the other or if they all feel good to you.