My only MTB is my old 26" XC race bike. Back in our prime (mine and the bike's), my typical setup was a 135 stem and narrow bars. In its currently set up as a singlespeed, and I went to a 100 stem with 630 bars, then to 80x720. It took me a bit to get used to, but I've found that I like the leverage of the longer bar for technical sections. If I was cruising non-technical paths, however, I would probably opt for the longer and narrower option, more like a road bike. FWIW, I have wide shoulders and prefer 44cm road bars.