Just to put this thread back on course...
The general consensus is, the 6.8 SPC over the 6.5 Grendal.
How about barrel length? 16, 18 or 20" length, or does it really matter? How about twist? The ones I'm seeing have a 1-9.5 twist rate? Is there a optimum bullet weight for that twist rate or does the 6.8 even come in different bullet weights?