I have both. Both are great. Both will do the job. The 6.5 has better BC bullets so it has a very slight edge for LR, but if you cannot hit it with a 6.8 having a 6.5 won't help you any. 6.8 brass is easier to find, but if you are not an anal person that has to have the headstamp match the caliber, a trip through a sizing die turns 7.62 x 39 brass into 6.5 Grendel brass. While some fire form it before working up loads it is not really necessary from what I have been told. I have had several people tell me that they see no difference in loads fired from straight 7.62 x 39 brass after a trip through the sizing die and those that were sized then fire formed. I have just gotten into making my brass from 7.62 x 39 brass using Winchester cases so I cannot say for 100% certainty but I trust the guys who have told me not to bother fire forming before doing load work up.
Bullet selection is good for both. Lots of load information. There is not the multiple chamber conflict with the 6.5 as there is with the 6.8 so IMO if you do go 6.8, get the 6.8 SPC II, 6.8 DMR, or the 6.8 x 43 chamber. Get a 1:11 to 1:12 twist, 4 groove, and a barrel maker I highly recommend is AR Performance. H has done more work with the 6.8 than just about anyone. Two of my 6.8s have barrels that are from the now defunct Cardinal Armory (Kotronics) and one has a DPMS M4 profile barrel. The DPMS is a 1:10 but has been rechambered to 6.8 SPC II. The two CA barrels are 1:11 and SPC II chambers. All shoot the old SSA combat loads (the real combat loads, not the ones they are now loading which have been loaded down) without any problems but I did not shoot those loads in the DPMS barrel before it was rechambered. The 1:10 twist does not seem a problem with my barrel but I cannot say about others. Be sure to check 68forums.com for more info on the 6.8. I have not been on the 6.5 forums in ages because those people are worse than the 6.8 people to a factor of 10. They want to run anyone that even looks at a 6.8 out of town on a rail while being tar and feathered.
If I had to choose one over the other, I'd choose the 6.8 because of the larger acceptance of the round, more companies loading for it, more information available for it.
By the way, my 6.5 has a medium profile LW 20 inch stainless match barrel with the .295 CSS chamber and one of my 6.8s has a medium profile 18 inch SPR barrel SPC II chamber, 1:11, 4 groove, and chrome lined. The 6.5 is slightly heavier profile. Otherwise the two rifles are almost identical except for the color of the furniture. One is as accurate as the other but I have not done load work up in either yet. So far I have shot Wolf (which I advise you to avoid) and Hornady ammo through the 6.5 and Remington and SSA through the 6.8s. Both of those rifles will hold 1-1.25 MOA with the loads it likes best. I've been doing too much load work up with some other rifles for the last year and a half so those have been sort of neglected.