2.1 is better for music because most music is captured at 2.1 and not 5.1. so really, the system would compensate for it and create a "pretend" 5.1 experience.
if you dont care too much about surround sound, i would go w 2.1.
now if u were watching movies/playing games, you would go for a 5.1 experience.
i have 2.1 M-Audio AV-30 monitor speakers and they are beast. the bass is brilliant in the sense that you can feel it hitting my desk softly.