You're talking about what is for all practical purposes the same cartridge, only one has a .257 bullet and the other has a .264 bullet, a whopping .007 difference.
Find the gun you like and let that drive your decision. In the unlikely event that you find a model chambered in both, buy the one the dealer has in stock. If he has both in stock, buy the one with the nicest wood.
I'd go for a Tikka T3 in 6.5x55. I don't think it's available in 257.
