I vote .260, largely because virtually all the factory ammo available is about as hot as good Swede handloads. And I think the cartridge really is catching on, and will be around in the future. Not that there's any flies on the Swede, mind you. I used to own one myself.
I read John Barsness' article on 6.5 rifles a couple years back in either Rifle or Handloader Magazine. He didn't do any huge development of handloads, but as I recall, his data showed a slight velocity advantage to the .260 with bullets up to 120 grains, a 50 or so fps advantage to the Swede with the 125 to 140 grainers, and with the 156 and 160-grain bullets, the difference on his chrony was 1 (yes, one) fps between the two cartridges.
I also have a soft spot for the .260 because of what it set out to do, and does so well....update the performance of the 6.5 Swede and standardize the round in new rifles, so standard factory ammo doesn't have to be kept at low pressures in deference to the century-old rifles so chambered. I believe there are Light Magnum 6.5X55 rounds out there, but they are only about equal to the regular .260, and in my limited experience, harder to find locally.The .260 offers sensible performance perameters instead of trying to be the latest red-hot velocity king.