Re: 260 AI ?
I am using 260 Rem now. It replaced my 6.5x55. Performance is very similar. Both running 139grn Scenars at 2800fps. The up side with .260 is the .308 Win bolt face - lots of options if you get tired with it. The downside is potential neck doughnuts if you cannot seat the bullet out of the neck shoulder junction that depends on your mag length. I seat deep but neck turn and have not had doughnut problems so far. R&P makes brass but I necked up .243 Lapua cases. Next time I will try nrcking down Norma 7-08 brass.
6.5x55 has a lot going for it with 26 degree shoulder and a nice long neck, but really needs a long action and is not a common .308 Win head size.
On 6mmbr there is a review of Terry Cross .260AI which he is doing well with. The downside of that is less barrel life.
With .260 I expect 3K shots before I will notice the difference, with .260AI from what I have read about 2K. but depends on application BR, F class hunting etc I guess.
Not sure if the 40 degree shoulder would create more doughnut problems or not, in one of David Tubbs books he thinks the steeper shoulder creates more doughnut problems.
Good luck with what ever you decide.