Foobar is fantastic for managing your library, but to unleash it's power you need a higher level of technical expertise than to use MM effectively. I does have a smaller footprint too, which I like. MusicBee is very similar to MM with some real nice playback features. MM is more flexible and MM 4.1 (Gold) adds significant features that MusicBee lacks.
One small thing about MusicBee which worries me is that developer is doing so much development for free (I figure he must have a daytime job ) that he may burn out or get fed up.
MM, being a business, has a better chance of being there long-term. I like MM gold best overall and it really doesn't cost that much, especially if you are patient and wait for one of the occasional promotions. If you are a penniless techie, go for foobar. If you are just penniless (or cheap
) go with MusicBee.