Go with the smallest cartridge that can do the job you need done, the more powder you burn down the hole, the less barrel life you will have, If you were well versed in ballistics, what affects a bullets flight, and how to compensate for them I would say get the biggest, baddest, flattest shooting MoFO you can find, but its no use burning up a barrel before you learn the capabilities of the cartridge along with your limitations.
The 7mm/08 will more than kill a deer at 400 yards, with a good bullet will remain supersonic beyond 1000, the only difference between it and the 7mm WSM on a long shot is a couple clicks on the scope knobs or 1 more hash mark on the reticle if you have the accuracy.
your talking about a big money chassis? you gonna build the barreled action to put it in? factoryrifle will normaly shoot 1.5 moa with factory loads, that's somewhere in a 6" circle at 400 yards, now if you wobble .00697" at the muzzle (yup just under 7/1000's of an inch) with that 1.5 moa shooter, you just missed a deers vitals. just something to think about.
RR

Another thing to think about is targets. When I am practicing with a rifle I already have dialed in for a particular round, I bang on an 8 inch "gong" out at 400 yards (my maximum allowed range for game). The 8 inch gong represents around the size of the vitals for a whitetail. Gong, can of quick drying spray paint and you are good to go. When I want to stretch my legs and bring out the .30-06 I have a 12 inch gong that I take out to 600-700 and a 16" that I take out to 1000. The Steel targets gives you visual verification of a hit at range without needing a spotting scope. 200 yards and in I use paper targets.

Ridge made a couple great points, but I wanted to be sure this one wasn't over-looked.

Long range shooting skill can be developed with a 22lr or 223rem, and doing so will save you a lot of money.

Jumping into a 26 Nosler, 6.5-284, 7mm Rem Mag, or WSM, or other hyper magnum without first developing your skill level is like buying a Top Fuel drag racer before you get your driver's license.

Out of the two, I'd prefer the 7 Rem Mag - and I know that it's not on your list. The 7mm WSM would be my choice off of your list. The 7-08 is a fantastic round, but it's a V6 in a class of V8's for super long range.

Added to my never ending reading list. Never a bad thing. :party: