myfault, I'm just gonna be honest with you, and you can take what you want from this post. I would personally stay away from the .260. First off, just from what I have seen even posted here about the .260 is does NOT have better ballistics than the .308. This is funny for me to say, because most people who have been around know that I'm not the biggest fan of the .308. From some of the data that others have posted and some of what I have seen on Chuck Hawk's website, the .260 is usually starting out at a slower muzzle velocity of the .308, with a bullet that weighs less.
http://www.chuckhawks.com/rifle_ballistics_table.htm You can see there where he lists the .260 with a mz of 2750 with a 140 grain bullet. The .308 he lists at 2820 with a 150 grain bullet, and 2620 with a 180 grain bullet. This means that a bullet that weighs 40 grains more, is starting out at only 100 fps slower than the .260's heavier bullets that it shoots. With some of the lighter .260 bullets, you might have better "long range" ballistics, but with some of the heavier bullets, not a chance.
Realize that there are kind of 2 thoughts on wind drift. You can shoot a light bullet faster, which means that it will have a shorter time of flight, and therefore the wind will have less time to affect the bullet. You can also fire a heavier bullet slower, and although it will have a longer time of flight, because it is heavier the wind will not push the bullet as much as a lighter bullet that had the time time of flight. There is also kind of an inbetween ground. Now the .260 with a 140 grain bullet, is still leaving the barrel slower than a 150 grain bullet from the .308, meaning it is lighter and it is moving slower. So it will DEFINATELY not have better long range ballistics.
If you are talking about hunting, a heavier bullet will hold onto kinetic energy out to longer distances. I can assure you that .308 ammunition, especially match grade ammunition will be MUCH easier to find for the .308, than it will be for the .260. I personally think that there are other cartridges that are better than the 6.5 grendel for long range shooting. I would probably take a 7mm-08 over a 6.5 grendel. If you're just looking at .260 vs .308, I'd say go with the .308 all day long and don't EVER look back. If you want to consider some other calibers, I'd be more than happy to talk about them. If thinking about the .260 vs the .308, I say the .260 is more of a fad, and a fun new cartridge to play with and say that you have and nobody else does. Go with the .308 if these are the main two options.
P.S. If you really were thinking about the 6.5 Grendel, I'd say go with a .243 over it. I'd take a .243 AI over the 6.5 grendel any day of the week. The .243 fires the same weight (sometimes) heavier bullets faster than the 6.5 grendel. With some of the heavier bullets, the Grendel might have a slight edge, but not much. You could still beat the 6.5 Grendel with the 7mm-08, and it would be just as easy to get ammunition for. .243 ammuntion is also MUCH easier to get the 6.5 Grendel. It's your choice, if it'll help you, I'll even post up some of the numbers so you can see it for yourself, and you can also do some research on it. Sure it's fun to say that you have some toy that everybody wants to hear about, but if it doesn't shoot really any better and costs you more to shoot, why buy it? You might see some loadings of the 6.5 grendel (handloads) that are going to be hotter than the factory loadings that might influence you. Just realize that if you compare apples to apples and compared some of the hot .243 loads, they're gonna put the statistics pretty close to where they were with factory loadings.