>>1229305mtb as you know is downhill for the risky parts and the gaps are smooth transfers if you land them.
bmx on the other hand is a lot of level ground and table top stuff on concrete. The falls are pretty bad because you fall at closer to 90 degrees and on cement.
So i would say, yes bmx will hurt more, if you fall on a mtb youre more likely to get road rash and slide burns which can be minimized with gear.
Bmx however will lead to impact crashes which gear is not so good at minimizing (except helmet).
