[Fargazer]

.308 is a commercial round based on a military based cartridge (or vis versa, but the point still applies), so there's a lot more availability of ammo for sale. It's also been around a while, and is used as a long range precision cartridge by both military and police. All that adds up to a LOT of data, bullets, and powders for the .308 round.

The .308 is going to require an AR-10 pattern platform, which tends to be more expensive than the AR-15, and less standardized. If you go that route, consider getting something that uses the Magpul SR25 / DPMS magazines; that looks to be the most widespread, and Armalite now makes rifles that use those mags also.

The 6.5 Grendel is a cartridge developed for long range shooting out of a standard AR-15 platform. Ammo is limited in selection compared to the .308, but that would be true for almost anything compared to the .308. The pricing isn't too bad, especially given Wolf's entry to the ammo market. It also has recently become a SAAMI standard, so more people should be making the round soon, which may drive the price down on the ammo.

One point of confusion on the 6.5 Grendel is the various chamber configurations out there. To my best understanding, Bill Alexander held the copyrights to the 6.5 Grendel, but let other people develop the Grendel without charge. Alexander Arms then got a new majority stockholder, who tried to make money off of it. Since the copyright was only for the name, Les Baer and others made "clones" with slightly different shoulders and chambers that would still work for the cartridge, and named them differently (such as .264-LBC). With any luck, now that Bill gave the copyrights to SAAMI, that will all shake out, but it is something to think about.

I'd recommend basing the decision on what you've got now. If you don't have a .308, go that route - lots of data, powder and match ammo available, and you can use the round in any number of competition sports. If you already have a .308, then look at the Grendel.