I got my Buffalo shirt way back in '96 and it's been fantastic, one of my favourite bits of kit. After many years of (ab)use, the pile is wearing thin and it's a little tight around the shoulders, so I got a Montane Extreme Shirt as a gift last Christmas. On paper the Montane looks so much better: hood included, better finished and more features. In practice I prefer to keep using my old favourite. Reasons:
Montane sizing only goes S, M, L, whereas Buffalo has chest sizes: Montane S is just about too small and their M is a little too big for me.
Montane has loads of extra features (drawcords, etc) which makes it heavier and bulkier than the Buffalo.
Chest pockets on the Montane are not as big on the Buffalo.
I found the Montane a lot warmer than the Buffalo (remember that a lot of the Buffalo pile has thinned); this can be seen as a good thing, but I've found it too warm to use in the conditions I've found myself in this year, therefore the Buffalo is the one that's in my walking pack.
Don't get me wrong - I like the Montane and it's really well made. I'll try to get some use out of it this winter, but I must sadmit that I'm still very fond of my old Buffalo (she's one of my five favourite things!).
Hope that helps.
Claudiasboris