Both good rounds. The WSMs are amazing triumphs of engineering.
I look at it this way, however. The benchmark standard for a deer cartridge is the .30/30, which has probably taken more whitetail than any other round in history and quite possibly more than all the rest put together. It's certainly good out to 100 yards. So take that as the minimum energy required.
The .270 Winchester can produce the same KE out to 400 yards that the .30/30 can at 100. The .270 WSM can do the same at 500 yards - which is a significant increase.
On the other hand, I cannot reliably put my first round into a dinner plate (the target zone for Bambi) at 500 yards. I'm not even going to try because there's too much likelihood of gut-shooting it or something. Indeed, 400 yards is pushing it.
So, to me, while the WSM has better ballistics than the .270 Win, it's an advantage that I cannot use.
And, as NSHunter1254 has so correctly noted, at distances less than that, dead is dead. I don't need the extra power to do the job.
Does that mean that the .270 WSM is useless? Far from it. If you really want the LCF of carrying a WSM or if you're one of those 1 in 1,000 marksmen who can really use it to its fullest, great. But in either case, you're paying penalties in terms of cost, recoil, noise and so forth. And if you drop your ammo overboard moose hunting in the boonies, you're far more likely to be able to find .270 Winchester in the local store than .270 WSM.
My two cents, anyway.