I have built 4- 26 Nosler rifles for customers now and loaded and shot all of them. The 26 Nosler is a beast of a round. You can push a 140 grain bullet 3300-3350 fps. But it takes about 90 grains of slow burning powder to do so.
The 6.5 Wby can be push a 140 grain about 3100 fps, and do it with about 67-70 grains of powder. Brass is easy to find, since you already have some.
Since you already have lots of 257 Wby brass, I'd go 6.5 Wby, unless you just want the crazy over bore speeds of the 26 Nosler.
When I spoke to Frank Green at Bartlein barrels, he said the provided all the test barrels for Nosler and some other mfg's. They were getting about 400-500 rounds out of a barrel in their testing. That's not very many rounds. It is a hoss of a round, but has it's draw backs.