For about 5 years in the 1980's my go-to deer rifle was a M1952 Mannlicher-Schoenauer .257 Roberts. With it I killed a handfull of Pennsylvania bucks and a small pickup truck load of Mulies and Pronghorns in Wyoming. 100gr. Norma (which you could still find back then) @ 3000fps. A very pleasant rifle to work with, and that load killed with alacrity (but all shots were under 200yds. as I recall).
I then snatched up one of the first M70 FWT's in 6.5x55 and proceeded to kill another pile of deer-sized game, using 129gr. Hornady's and Nosler140gr. Partitions. Again, a very pleasant, accurate rifle to work with that also killed like the Hammer of Thor. (Sorry, had to slip that in!)
My point is that through strictly empirical observation/usage I can't honestly tell a mouse's breath difference in practical performance. Close your eyes and pick one, if you can't have both. Or get a 6.5x57 and split the difference!
