As others have stated, as much as I like the 7mm RM ( I have two), I wouldn't do a 7mm RM with a short barrel, 24" is my minimum for the 7. Based on your specs, I'd choose the 308 Win for this build.
If he weren't running a suppressor, a 20" magnum anything wouldn't make that much sense, but with a suppressor, a 24" barrel makes equally little sense. Short barrels are the standard for people who run cans. You lose *some* velocity but 70 grains of powder in barrel length X still pushes a bullet much faster than 45 grains of powder in the same barrel.
When I chopped my 7WSM I lost 75-80 fps. It is still 400 fps faster than my 7-08 shooting the same bullets at the same (close to max) pressure from a similar barrel length.
I happen to dislike short unsuppressed barrels but that isn't what we're talking about.
