I am a real old guy and have a 6.5-.308 because the .260 was not invented at that time - need more trimming for 7mm-08 & .243W brass. I still have that old Ruger M77 rifle. It has a beautiful Hart, 9 twist barrel and likes 123 Hornady ELDM's and any one of many different powders. It has also shot down silhouette chickens at 1000 with the 140 Sierra MK and H4350. I see no need to get a 6.5 CM.
If I planned to get a real shorty action/magazine rifle and achieve a perfect fit, I might consider either the 6 or 6.5 CM but my ancient rifle still works. I think the 6 & 6.5 CM are here to stay. I would not be surprised to see a military application for the 6.5 CM, 123 gr. 6.5 FMJ bullets are available.
I have had much hunting success with the .25-.308 aka .25 Souper and have shot several deers with the 100 grain Barnes TX (hollow point bullet) and many rodents with the 85 & 100 Nosler BT using a wide variety of powders. I still have the RCBS .25 Souper F/L die set and would sell it at my cost plus shipping (real cheap - it needs & deserves a good home).
I think the 6.5X55 with its funny .480 rim diameter is kind of a peculiar cartridge and have no plans to get a 6.5X55 because of the unavailability of cheap brass. 6.5X55 rounds don't fit inside my real old Rem 700 bolt, the extractor won't snap over the .480 rim - probably just my rifle as I know there are other Rem 700's that fit .480, 6.5X55 rims.