6.8 Western is closer to the .270 WSM. It is just the WSM case shortened .08" and a faster twist barrel. I'd probably chose the cartridge based on what I can fit in the magazine for COAL and still feed. Your magazine length will be your biggest limitation for either rifle, but the Western design at least took that into consideration.
While .270 components might be plentiful, will the heavy for caliber high BC bullets 165+ be plentiful or seasonal? I'm sure 6.5, 7mm, and .308 will always have more options for high BC bullets. Manufacturers are starting to offer high BC 6.8/.277 bullets, but they have a long way to go to catch up to the others.
Now I'm a die hard .270 Win guy, love it and it'll be the one cartridge I'll always own. I don't see much need to improve on it, and its proven track record. I've found it to be quite capable with the bullets that work in a 1:10 twist well beyond 400 yards.
It'll also depend on if you're building or buying. If you want an off the shelf rifle the 6.8 Western will be the easiest to aquire. You're simply going to have to build the .270 Win to get what you want.
