I have a 257 but not a 6.5-300.
Depending on what you need the rifles for I might be inclined to simply use the 6.5 as my "do all".
I hunt moose with a 7.08 (Eastern Moose). It is moose deadly shooting a 140 grain A-Frame or Partition at a measly 2700 fps. The 6.5-300 would push that same bullet (140 A-Frame in 6.5 of course) at 3400 fps.
While I would use the 257 for moose in a pinch I see it more as a long range, deer sized caliber, but for deer I shoot a 243. My 257 is one of those "I want that" rifles for me.
Unless you are hunting some African or Dangerous game, I personally don't see what a 300 Wby would do for you that the 6.5 wouldn't do for you.
But I have a couple lockers full of rifles so have no aversion to adding another rifle "just because"