The Grendel uses a 123 Lapua Scenar bullet that will definitely have longer legs than any of the 115 grain SPC bullets.
Jim Brockman can chamber a barrel for you.
These folks sell replacement AR uppers, brass, bullets, and dies for the Grendel:
http://www.competitionshootingsports.com
I don't know if either bullet will still be supersonic at 1000 yards. Alexander Arms or Competition Shooting Sports will be able to answer that question for you.
JCN

The selection of long-range .277" bullets is, er... not.

The idea was to efficiently punch thru body armor at 600m, IIRC. Either will do that surely, but what on earth made them choose that silly caliber? (No offense, Jack!)

Once the uppers are more available, you're definitely going to get more bang per buck out of the SPC. If you're looking at the AR, I'd easily vote SPC, but for long-range (600-1000) not sure I'd pick that platform. In a turnkey, the chambering choices are wide open, leaving both of these somewhere below the top of the list.

I agree it's splitting hairs, but the split looks something like $ vs bullet selection. Who knows; if the cartridge takes off, we may see some match bullets for it.

(added

If you really want to play, get an AR10 and thread a 260 Rem bbl on it. Now you're talking.

Here is a link to some 6.5 Grendel info including a ballistic chart that compares the 6.5 Grendel to the 6.8 SPC, 7.62 NATO and 5.56 NATO. Feel free to download under section 4 on the linked page.

6.5 FAQ Sheet

The 6.5 ballistics are clearly superior.

Good post TX!

Shows why you just have to love 6.5 mm stuff.

cheers
seafire