Bose
I prefer the x to the a20. It feels like the a20 has a tighter clamping force despite it's specs on paper. That's the deal breaker for me. The a20 sounds marginally quieter and the phone function is good but the control box is bulky and so needs to be clipped to your pants or similar in order for it to not weigh down the earphone.
I have the privilege of chosing to use the x or the a20, and I'm going to stick with the x.
I haven't tried the Sennheiser.