well, larger cutting diameter for one, especially handy for turkeys, and the other, is the mech. won' t plain as much, which can be caused by bad shooting form and also shooting in a cross wind, such as out here in the west on antelope or mule deer! As far as weather or not a fixed head would keep doing more damage in an animal than a mech head would, heck, I thought evryone that shoots fixed heads ALWAYS get pass throughs! So, apparently, with either type of head, the damage should be already done on entry!!