I enjoy the original series more as the games progress and Mega Man gets more abilities (sliding, charging, Rush power ups, etc.) To me, the more options you have in your arsenal the better and more interesting it is. (Side note...screw 9 and 10 for reneging on the progress of the series in name of being "retro.)
The sheer amount of ways you can take on any X game is awesomely staggering and keeps the replays fresh. With/without armor, e-tanks, heart tanks, boss order...not to mention the later games that have Zero as a playable character with his own unique upgrades. I think the X series is head and shoulders above the MM series.