I tried both out in a store (with all of the usual caveats) and came to the rapid conclusion that the Momentums sounded significantly better but that the Sony's were a lot more comfortable. So i boughts the Senns. I haven't been disappointed, I do think they're great. My ears are a little on the large side, so the fit isn't perfect, but I don't find the SQ to be affected. They still sound great several months on, and I am NOT disappointed when I swap between my HE500s and the Senns. I definitely made a good purchasing decision.
I was really impressed with the comfort of the Sony's however, they were like wearing a nice hat :-)