While I am a fan of the 6.5 Grendel, for most people, I think either one would work well. Inside 3 or 400 yards, I don't think there is really much difference. Out past that the 6.5 Grendel has the advantage. I also like how the Grendel is also more versatile in the bullet weights that you can use.
The proprietary nature of the Grendel and that fact it has been slower to catch on is a big downer for it. While I understand why they kept such close control of the product, I think they were overly zealous and would have been better off getting more out there sooner. When people can't get a product that does tend to turn people off. Of course the 6.8 had its issues for a while too with the different chamber specs and such.
From what I have seen, the 6.8 has the edge on popularity and selection, I don't think it is overwhelming. It depends what your other factors are.
While If I had to get one right now it would be a 6.5, it will be a while before I start getting some of the more uncommon calibers.