I stayed at both Music and Sports last year, and each resort had their own bus way more often than the shared. If we left the park at close, we never shared! The great thing too is that if you are waiting for the Music bus and a Movies one shows up, you can hop on it and be on your way. Each All Star resort is only a 5 minute walk away from the other ones.
Keep in mind too that the Pop is 50% larger than any All Star Resort. So you have that many more people to share a bus with.