Well, Deli says there's no need for debate about A SUMMER BEST; Boys & Girls is Boys & Girls - the AUBE Original Mix
For anyone who's curious though, the A BEST version of the song wasn't marked as "new vocal & mix" like the others, but there are indeed some differences, although very minor, iirc. I'll have to ask Deli what those are. But they haven't used it on any compilations since then, so I don't see why this one would be any different.
And Foyuki, I thought you meant the difference between the times as a whole, like 4:41 and 4:43. Don't mind me. But yes, Greatful days is the A BEST 2 version, with the longer intro.