They both start off kinda slow, but they're both awesome.
Jack Wall did the music for Myst 3 and 4, and although it was fantastic music, I just don't feel that it fit very well with the "feel" of Myst games. All of the other games have a rather quiet, ambient soundtrack, like this piece from Riven, which I think goes better with the atmosphere of the Myst games. But I like both kinds equally.