Between the two, 338 WM, no question.

But if you're going to use the 338 when you only go elk hunting (and it won't be that often), then go for another caliber like a .30-06, .338-06 or .35 Whelen.

True that you can use the .338 WM for anything, even whitetails. But you'll want to use more versatile cartridges like the ones mentioned above more than the .338 WM.