I have both, and I would say it depends on what you want them for, as a spare pair to sit in your bag for when all the gloves are wetted through then the buffalo cant be beaten, as they are so light weight and packable.
However, the extreme mitts are reinforced on the palm with a rubber type fabric, have a better velcro wrist fastener and one handed draw cord on the gauntlet. The combination of pile on the inside of the hand and primaloft on the back makes for a warm glove even when soaking wet.
If I could only have one pair for Scottish winter climbing, then it'd be the Montane extremes. The Buffalos make a very warm spring and autumn mitt though. Also add some elastic idiot loops to stop them being lost!