Two things:
1. The biggest single advantage to narrower tires is less aero resistance. If everything else is equal, fatter tires will have equal rolling resistance with lower, more comfortable air pressures. If you're looking for aero, a converted mountain bike is a poor starting platform.
2. I think that really skinny tires look goofy on a mountain bike.
Whichever you choose, I think you are going to LOVE the change from knobbies for the that use you indicated.