To avoid confusion I'll chime in (considering I'm one of the few people who were actually around when neomame was active).
NeoMame was just that.. mame with ONLY the neogeo driver enabled. No extra games, no unlocked extra games that mame didn't yet support, nothing.
As much as I would like to give you an explanation of why this totally useless build was ever created, I can't give you one. Probably the same reason cps2 mame was created. (So people who just couldn't wait one mame version for a game to be added could get it up and running early.)
Neomame plus is another animal. It would be considered an "illegal" build as it did unlock games.
As of now, all neogeo games are in mame save a few homebrew games you shouldn't bother with. The only reason you'd run a dedicated neogeo emulator at this point is if your machine couldn't handle running the games in mame, which is unlikely.
Neogeo cd games imho are essentially worthless. There are maybe 2-3 games that are cd exclusive, one of which is a sampler cd the other of which sucks, all of which are in Japanese and require the ability to read said language to be enjoyable. Don't get me wrong some neogeo games have special "home version" modes, but they are generally a lame variant of the regular arcade mode. It's just not worth it.