They are relatively similar, with 9 being generally more balanced in terms of acquired powers and levels, but 10 having the addition of easy and hard modes (the former decreases enemies and adds 'covers' to some of the many death pits and insta-death spikes) and 2 playable characters instead of 1. (although each has another playable character with DLC). Best of luck.
As for why XBLA only has 9 and 10, well, 1-6 were originally NES games, 7 was SNES, and 8 was PS1. I would imagine that has something to do with it.
It would be great if Capcom would re-release the Mega Man Anniversary Collection (and X collection) for the current generation though (they were PS2/GameCube/Xbox games), but you can find most of them on the Wii/3DS Virtual Console and some on PSN.
