I have a .257 & love the gun. To compare the two calibers ballistics is somewhat of a challenge though. I use 115 gr Noslers in my .257 when compared to the 130 gr .270 the .257 shoots a bit flatter but the .270 hits harder (more energy) @ 400 yards anyways. Just did a quick comparison on guns&ammo's ballistic page and it really depends on what load you are shooting.
When I bought mine I was also looking at the .270 WM but I already own a .270 WIN so I figured I would change it up a bit.
I found when buying any new rifle / caliber go with your gut feeling cause deep down you will already have your mind set on one or the other.
Have fun with your rifle whichever you choose.