I don't know that he actually used either of those rifles during the shooting spree. And if one of those rifles is a 308 and the other a 5.56 then I think the 308 would cause more causalities.
Chances are, some of the first few hits with a 308 at 400 yards would have sent the whole crowd running and screaming. Hits with .223, penciling through at that range, might have given the murderer more time to pick targets without the crowd knowing what was hitting individuals (cheering, fainting at the concert?). I doubt that he effectively hit many of them, after they were running and scattering, although there might have been fragments from oblique skips on the pavement.