I have some experience running a gas gun in PRS. I would stick with 6.5cm simply because it almost seems too easy to tune. We all know how finicky an ar10 can be to set up and run reliably, and while I've never shot or seen one in 25cm, I have and have had several in 6cm, and they can be really tricky to get them tuned where you'll trust it in a match format all day or all weekend long. The 6.5cm in my humble opinion is the easy button for ar10 cycling, even easier than the crusty .308. I don't really think the difference in those two cartridges will get you one more impact, so you're potentially inviting a more tricky gas system for no real gain. In addition, gas guns are rough on brass. I don't load ammo specifically for ar10's in prs matches because it's going to get trounced as often as not, and you can buy good bulk match ammo for 6.5cm cheaper than you can load it. Not an option for 25cm. Good luck either way.
