Plainly, anything one can imagine can be found online. But when a guy talks to manufacturers about their trends and observes the sales volumes, and then correlates local availability of products to those numbers as validating data, the trend is clear.
I’ve been building AR’s for customers for almost 20yrs, I’ve noted the shift in the last 3-5yrs in the calls I get about the 6.8 becoming more and more calls about 6.5 Grendel.
You also have to consider complimentary products in the market. The recent 6.5mm boom is a tide shift, bringing assurance of future supply of 6.5 bullets - which was already assured, to a lesser extent, by already stable and long-standing 6.5 cartridges. Equally, 7.62x39mm isn’t going anywhere, ever, so brass, bolts, and magazines will always be available for the Grendel, as the difference is 10thou more run in for the manufacturers on the bolt face. The 6.8mm shares bullets (kinda, due to limited mag length and disparate velocity) with the .270win which has already been fading in popularity in the last two generations, and its parent case is an obscure, obsolete cartridge. The fact Lapua makes brass for the Grendel and not the SPC is something in itself for some of us.
The 6.8 isn’t going to die off any time soon, but it’s certainly not leading the race these days. There are AR cartridges which won’t ever be mainstream leaders, like the WSSM’s, 7 Valk, 277wolverine, 25-45 sharps.... and then there are AR cartridges which will likely enjoy a relatively stable niche, like 7.62x39, 458 socom, 300blk, 6.5 Grendel, and 6.8 SPC. It doesn’t take much for any of these niche cartridges with proprietary cases, or obscure parents to simply slip off of the board from niche to near-wildcat, or even obsolete.
There are advantages for either, but as much as I enjoy my 6.8’s - which I just built another 10.5” 6.8 - the longevity is easier to see for the Grendel, and if you want to reach past 500yrds, the Grendel starts taking over on performance. Inside 500, the difference is moot, even on game.
If I were brutally honest, I’d skip both if I didn’t already own them and build a 6mm variant on the Grendel case. Something between the 6mm AR, AR Turbo, Fat Rat, or 243 LBC. I don’t know that I believe the claims of 2900fps with the 105-108grn pills, but I do know it runs fast enough to do what I need it to do for deer and hogs, and the slippery 105 Hybrid won’t fit in a 6-6.8 to be mag fed, so the shorter x39/Grendel case has a distinct advantage. One of these will be my next AR, and probably even be one of my next bolt guns - basically a baby Dasher with better brass availability!