Two diffrent ports of the MAME core. The latest version of MAMEoX uses the .72 core and MAME-X uses .61 or something like that.
MAMEoX support more games than MAME-X, but MAME-X plays some games better (narc, for instance. Huge diffrence. At least for me. =)). I have heard somewhere (think it's in the compatability listing for MAME-X) that in later versions than the .61 core the sound for some roms (golden axe for one, I believe) is broken.
MAMEoX is being actively worked on and MAME-X isn't.
If you are into MAME, then you should get both, if it doesn't run good in MAMEoX, try it in MAME-X. If it doesn't run well there, then... you are out of luck. =) Smile and wait for the next update of MAMEoX or (dare we dream? =)) MAME-X.
If you want to know what MAME is, then you should look at these web-pages:http://www.mame.net/http://www.mame.dk/
EDIT: I started writing this reply before Bender_Unit_1 replyed, just forgot to post it. Promise! =) This post has been edited by kvajnto: Sep 10 2003, 01:11 AM