.300 mag
.30-06

elk/deer combo rifle? IMO you bring an elk rifle 1st and foremost, and use it on deer.

a .300mag 180gr TSX/Nosler partition, will kill a deer no problem.

I'd say any elk rifle/cartridge would be my choice...

EDIT...so re read the question...
If you're gonna use your rifle on deer for the next 30 years and hunt elk maybe twice in those 30 years... no reason to buy an elk rifle to use twice when you're gonna deer hunt every year with it. Also btw down the road sometime you can buy an elk rifle if that time ever comes.
I'd certainly buy a rifle for eastern deer that could double for elk probably...ie 30-06, 270win/wsm, 7mm rem.mag., 300mag,

my short list of deer rifles that could double for elk would be...
any of the .270's
7mm Rem.Mag.
.30-06

I'd avoid the .25-06 for elk, the .243win, I'd prefer something larger than the 308 case, ie 7mm-08, 308, although the 308, 338federal, etc..certainly could take an elk probably at under 200yds, and would make for a good deer cartridge,

my top 2 choices I think would be .270 WSM and 7mm Rem.Mag.