From what I understand from a friend of mine who has a .270 Weatherby Mark V is that the cartridge is ballistically very similar to the 7mm Remington Magnum, but in .277 caliber rather than .284 caliber.
I suppose this comparison is much like comparing the standard .270 Winchester cartridge to the .280 Remington (both use a necked-down .30/06 case)... i.e., they're
BOTH great cartridges as is the 7mm Rem. Mag. vs. the .270 Wby. Mag. !~!~!
Since you already have a .25/06, I'd opt for the .270 Weatherby.

Strength & Honor...
Ron T.