I actually was just putting on new pants just now, and I can't recall which leg I chose. Pretty sure it was left, but on average, I can't say. (stuck in my head now, probably going to use my left leg forever now)
I am more balanced on my right leg (something I am trying to correct often), so my left leg is habitually the first to be lifted. Only once in a while do I try to pull a stunt with both at the same time.