Here's a few facts about the 260 and 6.5x55, based on my own testing and reloading experience:
1. Components for the 6.5x55 are EASIER to get than components for the 260. In this case, I mean dies, gauges, etc. For instance, Lee offers 6.5x55 case length gauges as factory standard, the 260 must be custom ordered. There is a far greater variety of dies on the market, from more die makers, for the 6.5x55. Full power factory fodder is easier to get for the 260, but in my experience, there's more Swede on the shelves overall.
2. The 260 handles 160 grain bullets just fine, even in 9" twist barrels. They have that same outstanding ability to penetrate, even all the way through my chronograph, hitting every major component on the way (I said naughty words when that happened). If you don't want to take my word for the 260's ability to handle 160 grain bullets, by all means get a copy of Ken Water's "Pet Loads".
3. Peter- The 140 grain Nosler Partition, at least the lots that I tried just a year ago, do not expand reliably or evenly when impact velocities drop below about 2100 fps. This uneven and incomplete expansion is what causes the bullet to tumble, and the partially expanded tip acts like a parachute on the front end of the bullet as it penetrates backwards. The rear core separates and goes about 4" deeper than the front core and jacket, turning into a little mini-missile that even shows some modest expansion. Muzzle velocity was averaging 2738 fps in my test loads, so I was running into problems with shots of 300 yards or longer, as well as reduced loads. (I tested 9 different .264" bullets from Hornady, Speer, Nosler, Remington, Sierra, etc., as well as 5 different factory loads in a T/C Encore 260 Remington with a 24" barrel. All loads were chronographed, and bullets were tested at different ranges and impact velocities by firing them into saturated newspapers from which all ads and slick/glossy pages had been removed. This allowed for a consistent, repeatable test.) The Nosler 140 grainers worked fine up where most hunting is done, but I see no reason to go hunting without a safety margin. What if the next lot doesn't expand well at 200 yards, or doesn't expand well in our puny blacktail deer? That's why I test bullets before I hunt with them.
4. If you do your part when it comes to shooting and hunting, most critters will have a hard time telling they were hit with a .264" (well, actually most of them are about .263") rather than a .277" bullet. With .264" bullets easily available as components from 80-162 grains, as well as bullet moulds and ready-cast lead bullets, it's a versatile bore size for any reloader.
5. The 260 Remington will survive, at least as a competition cartridge, since it's been around in wildcat form since the 1950's. In a perfect world, it would take a good size bite out of the .243" and .257" market, since a better range of bullets are available. I haven't noticed this happening, but my kids will be starting with either the 260 Remington or the 6.5x55, not the .243 Winchester, 6mm Rem., 257 Roberts, 25-06, or any of those other traditional "first hunting gun" calibers. There's too many elk in this neck of the woods to set my kids up with guns that, IMHO, don't quite meet my criteria for a big game rig.