In terms of a sniping rifle, probably none. Both should hold premium accuracy for more than 2,000 rounds. A rebarrel is cheap and quick.
Sounds reasonable. So they are keeping the old M-24 receivers/stocks?
Why not go to all new rifles in .338L if they want long range?
And Uncle Sugar is paying for it. I would think that the heavy barrels used on the .308's could have their chambers reamed to accomodate .300 Win. mag., but they'll brobably just buy new rifles.