any mountainbike can be ridden on the street and MUPs.
I find the fastest, long-wearing tires that perform well in the dirt are the Geax AKA or Saguaro. (hardpack or all-around, respectively)
if you have two sets of wheels, you can use slicks or semi-slicks on your MTB. another good choice is to have a second bike. a MTB with one set of wheels and wearing slicks is a sure sign someone bought the wrong bike, but as long as they're happy I'm happy for them.