Personally, I find a BMX bike to be a blast on MTB trails. Nice and nimble, for those twisty areas. I do keep my seat cranked up, so I can sit on the flatter areas.
If, you're going to get a mountain bike, you should probably get one that's properly fitted. MTBs are designed for a different riding style, than BMXes. You can't just set one up like a cruiser.