Endyn in the past has been known to shred belts. Its something that I have heard with no actual experience. Besides, i think the MVM is cheaper. All you got to do is make a list of the two side by side and the decision should be pretty easy to make.
Having dealt with both companies, I'd go with MVM. I bought my rods and piston rings from Endyn, and I got tuned by MVM.
Endyn has much more bad press surrounding it than MVM. (endyn supercharger anyone?)
Also the above, the MVM pully won't eat belts. When I went, he showed me his pulley next to the endyn piece, and you can see where the endyn piece is about 2tenths of an inch "off"--thus shredding belts. Also, to add flame to the fire (take this with a grain of salt, though, of course) MVM Jim says the only reason he designed his pulley was because Larry's kept shredding his belts.