The decision to go with 5.1 system always have to do with space. In order to truly appreciate a 5.1 system, you need the room to install and enjoy the true surround effects. 2.1 system on the hand are meant for congested rooms.
Whether its for music/movies, a typical 5.1 system always scores over a 2.1 setup.
In the above case, MMS460 boasts 100W of power whereas MX5021 packs 90W of power! Talking about features, MX5021 definitely has the advantage being THX certified.
But thats all it got. MMS460 scores with its superb highs and tight bass response. Plus, the price is really attractive.