mountain bike is overall faster, but sometimes bmx is better since it can take harder hits, it's smaller, and more maneuverable
It can be pretty fun to play hard trials with MTB though. Basically all my WIPs are MTB because of the added challenge of harder maneuverability with it.
Mtb for going through rock terrain or normal jumps, and bmx for trails and really rough terrain, more maneuverability and can take more of a hit
How i roll is BMX for trials and autos (no not the ones you see on here that are 10 seconds long and take no skill), MTB for normal tracks.
Personally im starting to prefer the BMX, Snoogley summed it up perfectly here: I like both, but all in all i just prefer BMX. Also i find that most shortcuts are done with BMX bcoz its smaller and can fit in tight spaces.
I can use BMX for ghosting better and some tracks. But otherwise... I would say just MTB for normal riding.
with bmx its often difficult to land things smoothly, so often I prefer mtb, but on trials, autos, and parks I love bmx
BMX just takes time to get used to because it is so different than the MTB. You just need to play with it a lot to get used to it; then it is no harder than MTB