Asking "what is the difference between them" lets us think you don't know anything about the different Neo Geo consoles. So, I assume you know what AES, MVS, CDZ, pocket color etc. are?

Anyway the neo-geo.com forums have a subforum dedicated to bootleg detection. Generally, if you open the cartridge and its full of generic EEPROMs and none of the boards have SNK (or whoever) written on them, it's probably a boot. (in general) There are some professional looking boots and some cheap, bootleggy looking real games. If a seller describes his cartridge as "100% working", it's probably a bootleg.

Memory cards usually go for $40-60 US on the Bay of E. I assume you know you don't really need one, all they do is transfer your high score from and MVS to an AES, and I think there are some functions arcade operators can use with book keeping.