If the action length is the same for the 6.5x55 and 260rem, I'd get the 260. You can load it with higher pressures and make brass from 243 or 7-08 cases if need be.

I own both and love both, but the 260 wins out because of the benefit of multiple brass sources.

I don't drink or Smoke. I spend my money on gunpowder and gasoline.