The difference in case capacity is around 3 or 4 grains of H2O, depending on the brand of brass that you're using, in favor of the 6.5x55.
I have multiple of both and can't see that there is enough difference, performance wise, to pick one over the other, particularly when you're looking at the same rifle either way you choose to go. The 260 might have an advantage in brass availability, since you can make 260 cases from a variety of easy to find parents brass.
I shoot, mostly, 100 grain Partitions in my 260 deer rifles and either 129 grain Hornady SpirePoints or 130 grain AB in my 6.5x55s.
