It depends a lot on what else you already own. There is a lot of duplication within cartridges. For example there is no reason at all to own both 7-08 and 308. Nothing one can do the other can't. Nothing a 30-06 will do that a 7-08 or 308 won't do.
The 6.5 CM and 260 can do the same thing if the 260 is custom built with the right barrel and if you handload. If someone owns a 260 and likes it there is little reason to change. But if you own neither there is no reason to choose the 260. It isn't by much, but 6.5 CM does everything a little better and does it with off the shelf ammo and rifles.
A 243, especially if twisted for heavier bullets is a great deer cartridge, but is border line on anything bigger. The 6.5 is the next step up that is capable on everything in the lower 48. It will kill anything a 270 will kill and at the same ranges. With recoil closer to 243 than 308.
