I'm surprised there's no one singing the praises of the "vastly superior" 6MM Remington.....Usually there is and that poster is conspicious by their absence.

Usually one makes the argument that the 6MM is more accurate because of it's longer neck.....which can't be confirmed in the minds of the .308 crowd.

About the only difference that is concrete is that the 6MM Rem has a slightly greater powder capacity and subsequently somewhat greater velocity. However it's very slight and for all practical comparisons, they're identical in performance. They shoot the very same bullets with the same accuracy potential and at the same velocity.
The local hardware store has one in stock and the other you often can't find at all. That's the difference.