I have Shimano Ultegra (although the shifters are older 105's) on one bike and Sram Rival on another. The Sram shifters fit my hands better although the new Ulterga's, I believe are now flatter on the tops and adjustable. Someday I hope to get a set.

The shifting motion is different for each of them. I went from Shimano to Sram so it took some getting used to, but now the Sram is fine. I think I have a slight preference for the Shimano, but that is probably due to the fact that I started with that system.

If I had a choice between Sram or Shimano, I think I would get a new Ultrgra (or even Dura Ace ) group. (However, I would love to try a Campy Record group someday. But that is a long shot!)