A lot of folks will take a stand and say "steel!" Characteristics such as durability, ride quality, and etc. are named as reasons for such. Durability and ride quality, particularly the latter, are effected just as much by frame design and geometry as the material, considerably more so in the case of the ride. So, while I am a big fan of steel, I would say that you're pretty safe with both aluminum and steel. Both materials can be used to make high quality, relatively inexpensive bicycles. Carbon fiber is less practical for a number of reasons, and titanium, while it has some highly desirable characteristics for a commuter (strong, high resistance to corrosion, etc) is pretty darn expensive. That said, there are folks here who commute on both of these materials, and love it. I'd say, go with the bike you like. If you like your MTB, definitely use it as a commuter! It's a great choice.