Over the counter ammo availability differs a great deal from one part of the Country to the next. Even one Walmart to the next.
In the S.E., the .260 has gained something of a toe-hold. Living not too far from where Kenny Jarret does business, I've even seen a few of his guns including a bug-hole shooting .260. A friend has used his .260 loaded with Hornady 154gr RN to shoot a deer on the other side of an 8" pine tree, just to see if he could do it. Bambi didn't survive the incident !!! Amazing penetration, and decent wound channel too! Though he says the 140gr bullets aren't much worse !!!
As far as performance on elk, I don't think you could tell the difference between a 140gr .264, a 140gr .284 or a 130-150gr .277". All are going 2,800 to 3,000 fps. I couldn't really tell much if any difference between the performance of my .338/06 and my brothers .308wcf, except he tote's 2.5lb less gun than I do !!!!! That btw is 25% less gun, and no perceptable difference. (Both will do the job with proper shot placement.)
Just get what you want !!!!
For you it sounds like a 7mm-08. For me, I'd go with the .260. The determining factor for me is, -I've never had a .264" gun. But, then, it would depend on what kind of deal you can get on the gun. That, and I already have 7mmMag, and 7mm-08 dies, brass, and bullets.......
Remember the manager at the Walmart can/will order you some of most anything thats available to them. They can/will even get you a Howa in 6.5x55 if thats what you want. At least they catalog them. A friend got his .243 WSSM in a Browning A-bolt from Wally. Took 8mos. too......... Wally even carries ammo for those, and all the WSM, and RUM's too, around here.
Personally, I'd prefer the .260 but there is precious little difference between it and the 7-08, unless you're shooting NRA Highpower, where the .260 holds a slight, but Match winning advantage.
If restricted to factory ammo though, the advantage if any exists, is to the 7mm-08 due to longevity. But if thats an issue, the .30/06 has both beat hands down!!!
The reason I haven't gotten a .260 is because it's too close to my .257 Roberts, which BTW "WILL!" go on my next Elk hunt. Why?, it only weighs 7.2 lbs with 5 rds of 120gr Nosler Part's. Much more significant than any difference in ballistic performance. Any of all the above cartridges will shoot "through" on a broad-side shot on your average elk.
I have a good friend that hunts them (elk) with a "Home-made" long bow............. and killed a 5x5 with it this past year, with a home-made arrow at that !!!
Why did I rechamber my .30/06 to .338/06 ?
I got "bored" with my .30 and got the barrel for $54.00. A single pass through the .338 dies and I've got a .338. Total cost of conversion of the /06 including dies was $180.00. And I've still got the other barrel and two sets of .30/06 dies too ! If I ever decide to switch back.
So, get both barrels !!!
AND......start reloading !!!!!
Friends can't believe the difference between factory .270 and handloaded .270's. It's like not even the same gun!!!!
You'll find the same with either the .260 or 7mm-08 vs factory ammo, except with the "Premium" ammo.
REMEMBER.......Hunting skill, marksmanship, and patience make much more difference than .020" in bullet diameter.
Too, in east GA, we shoot Hogs with .22WRM's. !!!
(WMA's outside of Deer seasons require "small game" cartridges, but this includes all Muzzleloaders !!!???)
Remember............shot placement, shot placement, shot placement........