It depends ... 5.1 speaker systems have 3 speakers in front of the listener and 2 speakers to the side/back of the listener. They also have a separate low frequency only speaker (.1) . A 2.1 speaker system only has 2 speakers in front of the listener and a low frequency only speaker.
Music is usually only a 2 channel source. But even with a 2 channel source, your surround sound decoder can do things to make it more enveloping.
Sometimes the choice comes down to the ability to wire in the extra speakers.