Either one will do the job, though I'd probably opt for a fairly tough, deep penetrating bullet for both like the Barnes TSX. I think that it would really come down to which rifle you shoot better and want to carry. There are a lot of people that will tell you the 270 WSM is too small for elk, but I figure that if the 6.5x55 Swede is good enough for European moose, for which it's used regularly and successfully for 100 years, then the 270 WSM will darn well kill an elk.

Actually, I think if I were you I'd take both rifles with me. Never hurts to have a backup rifle back at the truck in case something breaks.

Mike