I would stick with XP if that's your current OS. IMHO MCE is great but I only use it for a dedicated machine connected to a plasma in my bedroom. I use plain GameEx on my cabinet with a custom theme. Although if I had an option to install MCE or XP I would choose MCE because it's the same thing except it includes Media Center and that can only be a plus.
Again IMHO, In an arcade machine all you need is GameEx as it gives you DVD, video, picture, karaoke, radio, web browsing, rss feeds, webcam and of course mame and other emulators. What more do you need in a cab? The only thing that MCE has that GameEx dosn't is TV and a TV Guide, and TV in GameEx is being worked on now. Personally I can't imagine why anyone would want TV in an arcade cabinet, but I guess it's pretty cool to show off features to your friends.
P.S Your always welcome to visit the GameEx community forums
here. Come on over and say hello!