Just had the same problem two weeks ago. Go w/ fixed, I had mechs malfunction on 2 deer and i lost both of them. I just did a personal test:
http://forum.hunting.net/asppg/tm.asp?m=1143448
I found it was pretty easy to tune my bow, and i got my groups w/ broadheads to be equal to that of my field points. Why have more moving parts where something can go wrong. Plus, its hard to beat fixed for penetration. And as far as im concerned id rather have a complete pass through than a wider cut dia