Originally Posted by
Caliper
The smoothest tires for the MTB will be your friend. Knobby tires are going to suck energy. Also, a skinnier tire will be a bit smaller diameter, giving you a slightly lower gear.
A 15% grade isn't anything to sneeze at, so while I'm definitely a drop bar guy, I would recommend the mountain bike unless you are in good shape. (I'm assuming that your 200# comment isn't meaning that you are 200# of muscle...) Tires are cheap, just get whatever you can with a smooth-ish tread.
I'm actually in pretty good shape, just a big guy at 6'3 and mostly muscle (those hazy IPAs have gotten me in the gut over the past year) but like most people, could lose 10 pounds to get to a more ideal weight.