There is a link to an article in a Momentum vs Kinetic Energy discussion that talks about mechanicals and fixed blades. Its worth a read! I am by no means starting up that discussion or a penetration discussion again but that article had some good insights, just though you might want to check it out. Fixed blades for me, but I suppose if I were taking 50 yd shots I would like mechanicals as well for a little less wind sheer.