As the others said, no real difference. If I were buying, and didn't want to handload, I'd buy a .260; the North American factory ammo is more powerful than the 6.5 Swede. I've heard it said that the .260 doesn't do as well as the Swede with 160 gr. bullets, but I read John Barsness' article on the two rounds, and he found a 1 fps difference. (Not a typo...one foot per second.) He did find that generally, 140-grain bullets could go about 50 fps faster from the Swede, about 2800 fps vs 2750 fps.
The similarity in their performance has made a lot of naysayers question why anyone would produce the .260 at all. The best reason is that they could make a cartridge which could do anything the Swede can, with low recoil, and higher pressures in factory ammo, and which was not chambered in hundred-year-old rifles. Lots of justification in my books.