mame4all-pi is apparently the fastest, but if you have a pi2 i reckon you should use lr-mame2003 because:
– it’s libretro/retroarch, so uses all the shaders, auto configuration, etc
– it’s mame 0.78, so runs just about everything that the pi2 can run. mame4all has a fairly small set of games.
0.37b5 is for the mame4all emulators, so it depends on what one you’re running.
merged: if two roms share a common file it only includes it once. now the roms depend on one another so you need them all
split: each rom zip has all the files it needs