Since the elk is such an extremely dificult animal to put down with a bullet (I think they wear armor now) I would go with the 338-378 Weatherby mag. Heck I could take one at 700 yards if need be (Only with a perfectly placed shot right behind the shoulder) and for those really close shots it would be just enough to get through the shoulder bone to break it. I would probably still need a follow up shot though.
This, ofcourse was being a little sarcastic (sp). If I could have only one elk rifle, being serious, then I would probably use a 300 WSM shooting a 165 or 180 grain bullet or a 338 WSM using a 200 or 225 grain bullet. I don' t like the belted cases of the regular mags. That' s probably why I don' t have any magnums.
My next rifle will be either a 300 or 338.