Stereo for music hands down. I do have a very good processor: EAD ovation with Martin Logan monoliths and Logos with Levinson, Krell, and Classe amplification. I have done quite a bit of experimentation in this area. Steely Dan's 2 against nature DTS is very good in 5.1, but these songs in 2 channel stereo are much better. Better soundstage and far more natural. Now, downmixing a 5.1 channel to 2 channel--the 5.1 sounds better--it was designed to be played back at 5.1. But compare it and the same material that was recorded for and played back in 2 channel--the 2 channel wins hands down--that is if your 2 channel playback is set up properly. I have found that 5.1 channel can compensate for deficiencies in 2 channel set-ups. For example, speakers too close to a wall that can't create an image properly in 2 channel set-up, sound fine in 5.1 because the center fills in and the rears create some soundstage. (This would also hold true for speakers that don't image very well--I had this experience when I owned Klipsch speakers). So if you are experiencing an improvement of 5.1 over 2 channel, you might spend some time on improving the 2 channel set-up--sounds like your missing some really great music.
Stereo sounds better than 5.1?
I don't think so !!! More speakers, if done right (ie. the right pre-amp), you get, better imaging, better clarity, better everything. Nobody that has any "Good" audio experience can disagree with this. If you do disagree, YOUR WRONG. I feel sorry for all you are missing. End of story.