Re: .257wby vs .270wby vs 7mm wby
I have a 257 and a 270 Weatherby. Love both of them. If one had to go though, it would be the 257. As wicked as the round is, it's just not nearly as versatile as the 270, especially for the type of game you want to hunt. You can run 100 gr bullets on varmit's at 3700 fps, 130's at 3450 for Whitetail/Mulies/Caribou and 150's at 3250 for Elk/Moose/Bears/Plains game, etc.. In addition, you're a little limited on bullet selection with the 257, where as the 270 now has some viable long range bullets available, ie: 150 Accubond Long Range.
IMO, the 270 is probably the most versatile of all Weatherby Mag cartridges. Hit's darn near as hard as the 300 and shoots as flat as the 257. The 7mm is fine round in it's own right, but just not as versatile.
Good luck with your decision. Whatever round you decide on, I would look real hard at the Accumark to house it in.