I use 2 bolt (SPD MTB) on all mt bikes. I like them due to the cleat being recessed in the sole. It's not only easier to walk but I think safer as the sole hits the ground instead of the cleat. Also with road shoes at a stop it's easy for your foot to slip if you put the cleat down a a slick spot. That is what mad me start using them on all my bikes. I ride a tandem and with a full touring load and the stoker clipped in it can be hard to hold up if your foot slips. Another thing to think about is if you ride a mountain bike you can use all your shoes on any bike.
If you do go with 2 bolt there is a big difference in shoes.I have a pair of flexible shoes I like if I'm riding to a store or someplace close and plan to walk around. If you try to ride a little farther the cleat will push up into your foot. I never wear them for more than about 25 miles. I also have stiff shoes I use on most rides. I've done cross state rides, 24hr rides, long brevets etc. with them and never thought there was any disadvantage from when I used road shoes.