depends upon how much you want to invest, a nexus or alfine igh is very nice on a commuter. they have a gear range very similar to an 11-34 cassette. A SA three speed hub is also nice if you can live with the limited range. however on your mountain bike you would still need a chain tensioner with igh, this sort of defeats the clean igh set up. it is hard to beat the single chain ring 1xN set up for commuting. get rid of the front deraileur and shifter, install a chain watcher or fang, remove the unneeded chain wheels, put on a chain wheel disk or bash guard - a pragmatic set up. also suggest replacing the knobby tires with a touring type tire- regular schwalbe marathon or continental contact are good choices - they will improve your flat protection and probably add a kph or two to your speed.