I've used both the NICE 6500 and Kifaru Longhunter on 20+ mile one way sheep hunts...both are great packs. Really comes down to what you want out of features....that said I went from NICE 6500 to Kifaru and haven't looked back. But just cause the Kifaru fits my body better doesn't mean it'll fit you better. For starters:
The Mystery Ranch be about 25-30% heavier starting pack weight than the Kifaru Longhunter.
The MR NICE 6500 has more built in exterior pockets.
The Kifaru frame is 26" tall and will have working load lifters even without the packbag on it where as the NICE frame will not.
NICE 6500 is made of thicker material from what I recall (which partly accounts for more weight) so it is slightly tougher I guess you could say, though both packs are bomber. That said I've put small holes in both packs from general use/abuse.
Really can't go wrong either way so long as the pack fits your body. Both companies have killer customer service and will get ya setup with what your hunting style might be.
Hope that helps.