Both have good soundtracks. MM9 seems to have played it safer and resulted in a more consistent soundtrack. All of the themes are memorable and evoke the imagery of what they're going for, and I especially loved the Wily Machine theme with the sirens going off. It gave a serious big boss feel.
Megaman 10 had a riskier soundtrack with more variety. In some places it paid off, in others not so much. Strike man's theme tried to sound sporty but it just fell flat (as did most of his stage, and overall design, but every game seems to have its one weak link. Still it's the same soundtrack that gave us Solar Man's theme. I don't think either Wily theme measured up to the MM9 Wily machine theme, but they weren't bad either.