They are hard to compare, both are fun in a different way.
While MTB can be quite an adrenaline-rush, you get less of the runners-high.
Riding more climbs and less technical stuff like gravel-paths, dirt paths and stuff like that in the mountains on a MTB can be compareable to a road-ride.
Fast descends on a roadbike can be an adrenaline-rush as well.
However using the wrong bike for the job makes things a lot more interesting:>27,5+ hardtail without lockout on flat roads>700x25C roadbike on singletrail>>1303975
You don`t need mountains to ride MTB, even in the netherlands you can find some decent trails.>>1303986
DH is way less physicaly demanding than climbing, it is however way more technical.