Actually there are quite a few of them. svc, svcc, svcchaos, svccb, svcplus etc. (there are a few more, but I don't remember those) basically there aren't any differences. Most of the sets are clones 8svcchaosa etc), and few are hacks. SVCCHAOS is the original *correct* game dumped from a PCB. SVCB was dumped from a bootleg, has has a list of bugs that can be fixed with patches. SVCPLUS is probably the most common set, also a bootleg, but has all the hidden characters and bosses selectable, has bugs that can be patched, and some extra graphics in the title screen and whatnot that nobody cares about.