They are very similar in performance, but there are a few things that set them apart. The 6.5CM makes a little better use of the case papacity, it is a more efficient design and offers a shorter OAL. The 6.5CM is also the choice if your looking for plentiful match quality ammo. I will tell you from personal experience, the 6.5CM runs on the edge of the pressure spectrum, I have seen many blown primers and scary pressure signs using factory ammo/load specs. The 260 is a longer case, and if you can run a longer OAL than 2.80 (AOL of the 140 Amax 6.5CM), this is where the 260 starts to have a edge over the 6.5CM, be it a slight edge. I found you can get higher velocities and less pressure signs with the 260, and longer brass life if you have the mag length.
You really cant go wrong either way, they will shoot side by side. If your a handloader, I would opt for the 260 but I am a little old school. I run a 6.5CM POF gasser, mainly due to the fact they are limited to a OAL of 2.80 in the mag.
Kirk Roberts