Just by you saying possibility of elk and moose I'd go with the .300. remember you can always put a muzzle brake
on your rifle (which it sounds like you're already getting a custom rifle
so why not spend the extra couple hundred dollars on something that makes a magnum caliber very enjoyable to shoot?)
I've personally shot a 600 lbs spike elk with a .308 before and I won't do it again. I killed the animal and pretty quickly yes, but since then i went on a craze of too much gun for any particular animal.
As for your 300 Win mag con of eating up the barrel not really. You get into some of the more extreme .300's with reloaded ammo and then you might have to start worrying.
That's just small opinion maybe someone else will chime in with more experience shooting Elk with a .308, it can be done, i just prefer more gun [img]/ubbthreads/images/graemlins/grin.gif[/img]