I would recommend the Wii or PS3 versions. I believe M2 did the emulation and I was very impressed with the Neo Geo games on both consoles. The NeoGeo X was really off in my brief experience playing one.
Agreed. Also, the PSP versions run beautifully on the Vita, and you can get quite a few of the Neo Geo games there (not all though). I have Metal Slug, Metal Slug 2, Metal Slug Anthology, Samurai Shodown, SS Anthology, Shock Troopers, and a few more. Due to very compatible resolutions between the PSP and Vita, the games really work great on Vita.
Also I have the NGX, while not terrible they used a pretty cheap (aka not bright/rich colors, more dull) screen on the handheld (that's my biggest complaint). The joystick isn't bad and works fine on a PS3 when playing the official Neo Geo games that came out for the console. The dock, meh; output quality is not great, but passable (not close to the original).