If I were hunting elk and could shoot either of those rifles equally well and they were both the same weight I'd go w/ the 300 Mag.
That having been said, I hate recoil and don't own a belted mag, and wouldn't hesitate to take my 30'06 out after elk. It will, however, have to play back up to my 338'06 which would be my "go to," just as I think yours ought to be the 300.
There is a lot of talk here about long shots (>300 yards) at elk and I have no problem with them as long as the caveats met... that is, as long as I practice like a mad man during the off season and know my limits. My standard (when hunting deer-sized game) for that 5/5 into a pie plate at any given range.
