I'm late to the party, but may as well get my $0.02 in. Was going to comment when relevant, but with work and issues at home, I just found the thread again.
I don't like forming brass if I don't have to, first of all. Even necking down to 6.5 caliber, there are a series of steps involved, and shortcutting any of them will yield less than accurate brass. This to me is WORK. I have very limited time already. I don't believe A-square still makes 6.5-06 brass. Bertram or Quality Cartridge might for a hefty price, but from my experience, the quality of Quality isn't quality...
If whitetail and mulies are all you're ever going to hunt, any of the choices will be fine. Of elk are on the menu, however, and I could only pick one of the 3, I'd have to go with a .270. That said, it would be a custom with 8 twist, lengthened throat, and 26" length of barrel to optimize the advantages. Staying within the genre, a .280 AI would be even better.
If the choices must be stock factory or semi-custom commercial builds, and we must stay within SAAMI specs for each cartridge, I would choose a 6.5x284 Norma over any of them. The 6.5-06 is not superior to Norma's neck-reduced iteration of the .284 Winchester, and there is reasonably priced brass available at every major (and many minor) retailers by every brass manufacturer that catalogues brass for the 6.5x284. At standard SAAMI specs, the 6.5x284 will carry more energy farther than a shorter throated .270 or .25-06, and recoils LESS than the .270 Win because it will typically use a couple grains less powder to achieve better ballistics. There aren't as many rifles available in 6.5x284N as there was 10 years ago, but brass is readily available and only slightly more expensive than .270 Winchester, and about the same as .25-06 WHEN you can find .25-06 brass (not as hard now as it was a year ago, but still...).
My opinion in the 6.5x284 N is based upon experience with multiple examples of it against multiple .270 Wins and .25-06s. However, if I can build a custom how I want it, I would still run a fast twist .270 Win with match chamber and long barrel. I don't think the energy difference will be much different at any range either ought to be hunted with even with the BC advantage of the 6.5. The .270 can fling 170 and 175 (I think) grain bullets with just a slightly lower BC compared to the 6.5 long actions that will be doing so with a 140 - 156 grain bullet.
And if I'm hunting whitetail and mulies only inside 500 yards, the .25-06 wins. I still want a 26" barrel, though, to burn all my powder. I also need a 1:9 twist! You won't find that ina factory or semi-custom rifle unless it's special ordered.
Good luck this hunting season to all!
