If you don't reload forget the 300 RUM and wby. Limited selection of ammo and very expensive plus downloaded. If you reload buy which ever you want.

Though since you have a .270 already and assuming you'll keep it for deer, etc. I'd agree with the suggestion of looking at adding the 338 wm to the list. Perfectly suited for what you want as an elk specific cartridge and big enough to handle anything else you may be interested in the future (at least in NA). If your wanting a 1 gun battery then any of the 300's would be fine but personally either 300 wm or RUM.