I would encourage you to get away from the idea of one do-it-all rifle. Why one? You said initially that for the next couple of years you'll likely only be shooting at medium-sized game. So get the perfect rifle now for that game. Hunting the west generally means carrying a rifle around all day, so for me, a great medium-game rifle is lightweight and shoots an equally light-recoiling caliber that is still more than enough for deer/pronghorn out a long ways. 7mm08 and 6.5 CM have both been mentioned here, and I agree.
Then, once you get closer to that elk/moose tag, get the perfect large-game rifle. It will be heavier, I assure you (cue the 7RM/300WM/etc. debate). But don't spend the next couple of years hauling around that elk rifle until you need it.
My opinion, developed after making the same mistake.