Any is more than fine on elk.
If you limit your range to 500yds, then absolutely no difference to elk.
If you don't mind a bit more recoil and want the maximum energy, then go with the .30.
If you want a bit less recoil, then go with the 7mm.
I see no reason to go with a Short Mag in these calibers as the brass offerings are less, as are the factory loads. If you shoot a caliber this big, saving a few ounces on a shorter action for you rifle is likely not a factor.
Don't get either of these calibers and shoot, light for caliber bullets(under 160 on 7mm and under 180 on .30), as if you are then you might as well pick a different gun.
Me, I am very fond of my 7mm RM's shooting 168-180gr VLD's for elk. Never found an elk that could walk away from a reasonably placed on.