I love 6.5mm, got a bolt action Grendel and two '96 Swedes. great bullet selection, good enough ballistics for target shooting and all sorts of game. I love my Grendel.
That said, if I was looking for a AR rifle, I would go for the 300 Blackout.
For each caliber reloading is the best option due to the newness of each cartridge, and the expense/limited availability of factory stuff. Brass for the Grendel is not cheap, and I would cringe each time I popped off a round, wondering if I would ever find the brass. 300 BO can be easily made from 223 brass which is plentiful and cheap. Bullet selection is great for both.
The bolts in 6.5 have had some issues, so if going that route you need to get quality components, which equals more money.
I just don't have the money to have a good AR in 6.5. To me the 300 BO would be a lot easier on my wallet and I would actually get to shoot it enough to enjoy it.
The only reason I would go the 6.5 route is if I was planning on doing more long range shooting. I think the 6.5 has much better ballistics and would be better in this regard.
If money was not a problem, yeah I'd get the Grendel, but then I'd also have the funds to get another upper in 300 BO....