The 6.5 mm bullets are a happy mathematical combination of high sectional density and ballistic coefficient. They buck the wind, hold velocity longer, and penetrate well.
As a silhoutte shooter way back we wildcatted the 7-08 as an alternative to the 308. I burned up a barrel of a 7-08 shooting 7500 rounds through it. I shot lots of game with it. Then I wildcatted the 6.5-08 because you could shoot 120gr at the 200, 300, and 385 meter targets and shoot 140gr at the 500 meter rams. The 140gr takes the rams down with less recoil than the 7-08. A .243 will not take the rams reliably. Remington then brought out the 7-08 and the .260 which is a 6.5-08. I currently have three rifles in .260 and have shot over 9000 rounds at targets and game.
When we were wildcatting, my buddy shot a 6.5x250 Savage, showing that there is not much new out there because it is basically the same as a 6.5 Creedmore. Other guys shot the 6.5x55 and the 6.5 BR. All these calibers perform the same out to 500 meters. Which is to say they are all great! Low recoil, great accuracy, and deadly on game.
Obviously, I'm a big fan of the 6.5mm. I would be happy with a 6.5 Creedmore, 6.5x55, or .260. As a handloader I like the .260 because brass is easy to get. I can use .243, 7-08, .308, or .260 brass. The 6.5 140gr bullet really penetrates. It blows through deer and hogs. If you buy a 6.5 be prepared, it will become a favorite!