I have looked at this issue quite a bit and have done LOTS of research on this forum.
While using the same ammo/loads, many members here have kindly posted their chrono results from various barrels.
As an average and when shooting .308, the members here post an average gain of 35fps per inch of barrel.
So in your scenario, you are probably looking at 100 fps difference between those two guns.
I hope that this helps you make your decision.