6.5 easy. handloading is fast and furious, tons of diff brass, powder and rounds out there for max accuracy. can load bullets from 90 grains, up to 160 grain vlds' for 1000 yd accuracy. also , it is easier to find factory/milsurp 6.5 ammo from the store or gunshow, than it is to find factory 257.
plus factory 6.5 , from gunshows, runs about 10 bucks a box, for swedish or South american,
the 260, while it is as versatile as the 6.5 is still about 20 , 25 bucks per box. Should be a quick death. I would love to have it, but until they make it as cheap as 308, no one, is going to mass buy the rifles, or the rounds.