I'm going to go with 10.8 being slightly better than 10.9. I finally got a chance to take it on my test loops, and while there are some improvements in 10.9, the biggest regression i've seen is consistently stopping 10+ feet before the intersection (not every intersection, but 2 compared to 0 in previous version)....i'm guessing the new native intersection assignment code might have bugs in it. In addition, i've noticed jerkier left turns.