After the last decade, I am surprised and reassured that this is still a debate. I chose .260 Remington early in and after some time I began to think I bet on the loser. It works fine for me, but like the .244 Remington versus the .243 Winchester back in the 1960s, I thought it would not make it this far. It reassures me that there are still ardent .260 Remington shooters and fans.
It's easy. I can use two words to sum up the entire .260 REM vs 6.5 CM debacle:
Hornady Marketing
AIAW is right on the money.
The question between the two is not based on performance, at least not much performance. Initially I saw just as many .260s as I did 6.5CMs. This was because both calibers were relatively new to long range shooting. How Hornady handled the 6.5 Creedmoor lead to it being a clear choice in the market today. Don't confuse this as the .260 Remington being not able to ballistically compete with a 6.5, because they are very, very similar. They both launch the same weight bullets at very similar velocities. Even in this thread, users are giving a slight, but real, ballistic advantage to the .260 Remington. It is all about how the two were supported in the market.
The 6.5CM was designed from the start to be a long range cartridge. The .260 Rem was designed as a deer hunting cartridge. Factory loads for the .260 Remington were initially plentiful, but they were all hunting bullets that were relatively light and marginal accuracy for long range shooting but fine for deer. The 6.5 CM from Hornady was available initially only in long range loads, featuring the 140gr AMAX. Contributing to the success of this was the Hornady loads were very accurate, efficient in flight, priced competitively and readily available.
Back in the day, those that chose the .260 Remington, reloading was the only option for long range loads. Also, the factory rifles chambered for .260 did not have match chambers or fast-twist barrels because they were hunting rifles and the SAAMI drawing for the cartridge (from Remington) was for a hunting-oriented load. The SAAMI drawing for the 6.5 CM is already a match chamber with a fast twist rate. The result was .260 Remington shooters had to have a custom rifle built for long range and reloads, the 6.5 CM was available in some factory rifles ready-to-go for long range (like the Ruger RPR) and good factory LR ammo, ready-to-go.
A quick search for factory rifles and ammo available today in these two calibers clearly shows how the 6.5 has eclipsed the .260 Remington in the market, both as a hunting cartridge and a long range cartridge. Hornady nailed this market and now leads it.
I chose the .260 Remington early on (2010). I knew going in I was going to build a custom rifle and reload. It was a flip of a coin decision for me. I am still a huge fan of the .260 Remington and do not feel out-classed against the 6.5 CM. I don't regret the decision. I can still get .260 brass and dies. I will always shoot a custom-built rifle for long range, so no disadvantage there. I am glad to hear that I am not alone in this regard.
If you plan on a custom build, you can choose either one with no penalty. If you reload, the same holds. If you are not building or loading, the 6.5 CM is a turn-key combination that has made it easier and less expensive to make the leap into long range shooting. Today, the 6.5 CM offers more rifles choices and more ammo choices if factory is your preference. This is because of Hornady's savvy market support of their cartridge, not because it far out-classes the .260 Remington.
Just my observations over the last few years. To each their own.
Action Guy