Have both. Love both.
270WSM- Factory Win. Model-70 FWT
300WSM- Custom Win. Model-70/Hart, long action 300short mag.
If its typically Deer, Antelope, etc. & occasionally bigger game Id vote for the 270WSM. But, If Elk are consistantly on the menu I'd go with the 300WSM. Not that you really need to, as the 270WSM is a Fully capable Elk cartridge, but I like the versatility & availability of the heavy, high B.C. & S.D. bullets already available in factory twist barrels in the 30 cal. Especially where big critters, at a wide variety of ranges are concerned.
Like I said, I love em both, & wouldn't part with either one!
Had 2-300's, and still have a 270. All light and ultra light weights. Both capable rifles for elk, or anything smaller. I prefer the lighter recoil of the 270.