I'm not sure, but interested too.
I've been setting up my RP3 over the past week, after first trying Retropie on my old RP1 the week prior.
The Retropie wiki lays it out pretty well.. just for MAME they have 5 different PI emulator ports, and they say exactly which romset is best for each one. So I think you want to choose your emulator 1st, then get the romset to match it. Mame4All runs the fastest, but AdvMame is the most customizable and supports the most games.
Right now, I'm not seeing much reason to not use AdvMame, since there seems to be CPU to spare on the RP3.
On my overclocked RP1, AdvMame was still too laggy on too many games. Even a few of the old 80's classics. But Mame4All ran everything I cared about, at full speed except for a tiny few that frameskipped.
Now on the RP3, AdvMame has run every game I've tried so far at full speed. Even late 90's fighter games, and various Golden Tee into the 2000s.. stuff that is not in the old 0.37b5 romset.
Aside from the libreto\retroarch thing, I'm not sure what the pros and cons are with the other mame choices.