6.8 SPC might be a better choice if resale is important. Probably more universally accepted.
Grendel has more poop.
Looks like store-bought ammo is similar in price with more support for 6.8.....$1.00 per round......ouch.

6.8
http://www.targetsportsusa.com/c-93-68mm-remington-spc-ammo.aspx

Grendel
http://www.targetsportsusa.com/c-928-65-grendel-ammo.aspx