you can hack in your own system select in Mame, I believe they still have it in there commented out, and it becomes one of the options when you press tab. Basically Kawaks and Nebula are faster than Mame and only run Cps-2, Cps-1, and Neo Geo. They do run all of these well and have more video options than Mame, so to a fan of the games that are on these systems the emulators are a plus. If you're solely looking for Neo Geo games, run NeoRageX, its been around forever, and theres been a lot of hacked versions released for newer games. It will run on systems as low as my parents 333mhz celeron.
If your system can handle all of the games Kawaks emulates in Mame perfectly, then there's no problem with sticking to solely mame, but if you want the extra video options, MVS/AES and country selecter, and all those other goods you can go with Kawaks.
Btw, Nebula also supports multiple MVS slots, its a little buggy when selecting which game you have in the slots, like I think odd numbers of games will make the selection a little off, but it works fine. This would be a cool setup if you had a real mvs cart and wanted to run it just like the real thing.