I have my third 6.5x55, a Win 70 featherweight. I think that the 260 is almost as good but not quite. A bit less powder capacity and I think the cases are both strong enough the the 6.5x55 shades the 260. I think the 260 will go away as a factory round also. Not enough wisbang to it and Americans seem drawn to that. How good a cartridge is means little if it doesn't do a min of 3000fps or shoot a 250gr bullet!
The ammo issue is a non issue. Just take up reloading! Cases for the 260 can be made from the 243, 7mm-08, 308, 358. If your really bored, I think you can also make them from 25-06, 270,,,,,,,,,ya get the point?
The real bottom line is that both cartridges will fire the same bullet's at damn near the same velocity, which is suitable for most any game in the lower 48,,,,,,,,,,,,,,,and Alaska!
The draw back to the 6.5x55 is the rim size. If you for some reason decide to re-barrel a Swede to something else, it'll have to have the same rim size. An option to the 6.5x55 might well be the 6.5x57. same rim as the 308 I believe. To make a bolt for a 308 work for a 6.5x55, you have to open up the bolt face a bit. On the Parker Hale 1200 action I think I also had to have the extractor modified just a bit because of the larger rim.