All else being equal, a fixed blade will penetrate deeper because with mechanical, some of the energy that the arrow produces is used up to open the broadhead. The problem with using fixed though is that shooting form, and bow tuning has to be right on, or there will be some broadhead planing.
Have you ever watched Wild TV and saw how shallow many arrows penetrate? That's what you don't want. I've only used fixed broadheads and of the 7 deer and one 450 lb black bear that I shot, all were complete pass throughs. That's perfect performance, IMO.