So, I have played Mega Man 9 more, and now have played and finished Mega Man 10...
When it comes down to it, both are good games. Lots of fun. Overall, somehow, I feel that Mega Man 10 is the more rounded of the two, and I really like its bosses more. However, some of it comes down to preference: I think 9 has more of a sense of humor, while 10 is more serious in tone. 10 does feel more in line with the latter NES games, while 9 is more in line with the earlier. Hard to say where they would fit in the whole pantheon. Maybe 10 would be MM 4.5 or 5.5.