308 family brass would definitely be cheaper and easier to find than 284.
You didn't mention what you plan to hunt. I've killed about 75 big game animals with a 7-08, including hogs, whitetail, mule deer, antelope and black bear. Ranges were 3 steps (hog) to 500 yards (antelope). None of those were guided hunts - I did all the meat processing myself and I saw first hand what the wound entailed. At no point did I ever say, "Damn, I wish I'd brought a bigger gun." I've also hunted elk with a 7-08. They didn't cooperate for some reason, but I did so with total confidence.
I can't see how you'd be disappointed with a 7-08 (or the 260 or their AI cousins) but you have only yourself to please here. If you're really wanting a 6.5x284, by all means scratch that itch. The only real risk to a 7-08 (or 260) is you might like the new rifle's mild manners and efficiency so much that the .280 doesn't go out the door much.
