I have both a .25-06 AI, and a .257 Wby Mag.
I have always been a huge fan of the .257 Wby......However, after building my Sendero SF .25-06 Ackley Improved 40*, I have fell in love with this caliber. It is smoother shooting, less recoil, less muzzle blast, 10+grains LESS powder, and just as fast as the .257 Wby Mag.
I say go for the Sendero .25-06, and have your gunsmith open up the chamber to .25-06 AI 40*, and stock up on .270 brass.
I use .270 Win brass because during fire-forming, the neck shrinks right about .020" so I take the once-fired .270 Win brass, full-length size it without the expander ball in the die. Then run them through my .25-06 AI full-length die. Then I trim them down to 2.514", then chamfer and deburr the necks, then primer, powder, bullet....And you're ready to form some perfect 2.494" .25-06 AI cases.
Sounds like alot of work, but it's really not, and it worth it. However, you can always just shoot regular .25-06 brass through it, but it once it expands and the shoulder pushes forward, the neck will be about 0.020" short, which still falls within shootable specs.
Here's a picture showing a .25-06 AI brass I formed out of .270 Win brass...And a .270 round next to it.
Here's a .25-06 vs formed .25-06 AI. Notice the just slightly shorter height of the formed case.