Can't speak on the mt. bikes since I haven't had one, but for riding on paved roads and mild packed dirt trails I used a Giant Cypress. I'm 6'1" and started riding at 350 pounds. Highly recommend.
I road mostly on the road, and not on trails with roots sticking out or anything extreme, but I loved this bike. I would look into a good giant. Go to Cyclist Connection. They'll take care of you and will do free checks on your bike for life.
My buddy works for Specialized and swears by them so maybe they're worth looking into.