I'll start off by saying that we haven't actually started used RS yet.
: However, I did spend a lot of time considering MUS and RS before choosing RS, and I've spent a lot of time going over the RS materials in preparation for starting. We're easing into RS A but plan to spend some more time doing pre-math stuff first (Family Math, the old version of Singapore Earlybird, etc.).
I viewed the MUS demo DVD, and was impressed with what I saw, but decided against MUS, at least for now.
First, at this point (Kindy) I have no interest in DVD instruction, except perhaps for foreign language. I really like math and I want to help facilitate some of the ah-ha moments, rather than give them away to the TV screen. (Just my personal opinion on that, so please don't be offended if you feel differently!)
Second, MUS uses manipulatives that are unlike anything used in any other program. Not that this is necessarily a bad thing, but if MUS turned out to be a mistake, we'd have to start over with new manipulatives.
Third, I really like the way RS approaches math and especially manipulatives. RS emphasizes manipulatives that can "exist" mentally as well as physically. For example, a child will eventually construct and use a mental abacus (though I prefer to call it a counting frame!) which gradually replaces the need for the physical abacus.
RS's use of games in place of most worksheet-type drilling is really appealing to me. Though my daughter still likes worksheets, she's also a game lover, so I think the games will be a hit when we get to them.
Good luck figuring out where to begin. For what it's worth, I think you can't go wrong with either.