OK, here's some advice that nobody has given you.
I love the .308, but if you get that once or twice in a lifetime elk hunt, you will want the .30-06.
I love short barrels, but they do not point very well, the muzzles wander. They also make for a butt heavy rifle.
So, here is what I recommend:
Buy the Marlin, with a 22 inch barrel, in .30-06. You will be very happy with it.
After you have hunted with it for a full year, if you still feel that the barrel is too long, then simply have the barrel cut and crowned to 21 inches, not 20 inches. I love a 21 inch barrel, it is very handy in woods, and yet still allows the rifle to point smoothly, without a heavy butt.
Swampy: The 7.62 Nato was invented for use in machine guns, not the .308 Winchester. The .308 Winchester is the commercial hunting version of the round, with a thinner brass case, and significantly more powder, pressure and powder. I agree with you that the .30-06 is a better elk choice, but if an animal is properly hit, it will never know the difference between 2,800 foot pounds and 2,600 foot pounds, with a core-lokt or any premium grade bullet. :-)