Shimano for me. I guess it's what I've been used to and I don't like Double Tap (just not my bag, though I can see why people do like it) nor the feel of Campy (again, personal preference).

I'm on the previous generation (6600) Ultegra on both road rigs and I find them really comfortable and so smooth to use. Maybe when the next bout of upgradeitis hits, I will reconsider, though I doubt it