I rarely play from 7,000+ as that distance is usually towards the limit of my distance (assuming sea level, relatively flat course, etc.). It's OK occasionally for the challenge, but long, forced carries and very long approach shots wears you out after a while. I also think playing from this distance tends to screw up my golf swing because one of my consistent faults is swinging too hard and losing tempo, and always pressing for distance exacerbates this fault.
Conversely, 6,000-6,300 usually feels too short, for totally unsubstantiated reasons (ego, testosterone?). I do play from this distance, usually early in the season on my spring golf trip which includes guys with a wide range of abilities. Opposite of the above, I think this helps my swing in the early season because I can be more relaxed and not worry about trying to kill the ball.
That said, of your choices I gravitate more instinctively to the 6,600 yard distance. I used to never think about total yardage but would go by which set of tees - usally the second set. A few years ago I was playing one of the Bear Trace courses in TN and instinctively went to the second set of tees. When I looked at the scorecard I saw the distance from the tips was ~ 6800, so I moved to the back and had a blast playing from there.
Andy