Generally speaking you want the .8 OD because the .6 drops your rpm by almost 50%, 60% in fact. That can lead to the motor not being comfortable at a reasonable cruise rpm. To determine which is better in your case you need to know two things:
1. At what rpm range will the camshaft profile, induction system, timing, etc be within accepetable range?
2. What typical cruise mph are you going to go?
An engine that "comes on the cam" at 2500 rpm and your cruise rpm is 1800 is NOT going to be fun. Gotta a BIG cam? Go with more cruise rpm to match it's characteristics. .8, or 80% of cruise rpm at mph vs 4th gear (straight through, 1 to 1) will increase your rpm more than a 60% drop in rpm will. Which is better? Re-read above!
I would design the gearing and camshaft for best performance at the highest interstate speed limit in your area, not to much over that or you risk a ticket.