definitely like a short stem and wide bar more, gives you much better control and feel for whats going on with the front end. only reason i see to run anything more than a 50 stem is for fit issues. If i were more into long distance rides and close to XC stuff id maybe go up to a 70 or so.
that being said, the 120mm stem on my road bike is definitely nice, not only for fit, but it helped a lot coming from a shorter stem in regards to handling. feels dialed for anything over like 10 mph to 40mph corners on big descents. so your application definitely matters.