Wait for that almost perfect quartering away shot than it wouldn't matter if you use either. Take a shot at a big animal while he is running away from you than you may want a big magnum. Too cover all basis and hunt successfully a big magum should be used unless you know of somewhere the elk are thick and close by. Long shots at big elk require magnums unless you see one stand perfectly broadside or sleeping.
You haven't hunted elk much, have you?
Most of my shots at elk have been full broadsides at stationary elk under 300 yards. My longest, at 487 yards, was standing broadside. None have been running, one (my first, back in the early 80's) was walking slowly and broadside at about 100 yards. Most of the time the longer the range the longer you can wait for a broadside. The longest I've waited was from about 11:30 to sundown when i got a 350 yard shot, again on a broadside.
Using rifle you can shoot well is way more important than the cartridge you use. Daughter #1 will be using a .308 Win this year with 130g TTSX at 3045fps at the muzzle. I'm not the least bit worried that it isn't a big magnum as she shoots it very well.
