These calibers are so close that I quess I would let the rifle decide. I have not hunted much with the 7x57. I have used the 7mm-08 and the 280 quite a bit and the 6.5x55 a lot. The 7x57 would have a bit of an edge over the 6.5x55. Unless you figure things like some on this forum.
. Pick your favorite rifle and you will be happy with either.