Generally men. We know how the entire process feels personally, and can therefore tailor the blowjob from our own experience of what feels good and when what feels good.
Woman can be just as good as men, but that's less frequent. That's ok though, while men generally give better bj's nothing feels better then when a woman plays her ace card; slipping your cock in her tight and warm pussy. Bi or not, to me, that is the best feeling in the world and something a man can never do.
Don't get me wrong a good blowjob is epic, but for me, it's a 9 on the scale whereas sex with a woman is a 10.