6.5cm factory anmo availability and selection is better by multiples. It has unarguably become one of the most popular cartridges in the US, and that shows in availability and selection. Nothing wrong with 7-08, but anyone saying availability is just as good is either fooling themselves or has a lucky situation that others may not be able to count on. Its true you can find SOME 7-08 ammo almost anywhere, but ime its rarely a broad selection, so if your gun is at all finicky or you want something specific, you might be out of luck.
Functionally, they are so close that differentiating between them is 95% in your head. I sold my 7-08 and replaced it with a 65cm, mostly because it was finicky and I had a really hard time consistently finding the ammo I wanted, even online. I think if there is any functional difference it may be that the specs on a 6.5cm seem to make them a little less finicky.