If you can manage it, I believe 8.6 is better.
OS 9 always felt unreliable to me. Maybe that's because I only used it on a school network with about 150 computers, half of those 333 MHz iMacs, and the other half 1 GHz (a few years newer) Dell machines. Everything was bad (I'm not in that school anymore). The network was convoluted, the internet was always slow, and seemed to go down often on both systems. On the other hand, I have had no problems with the extremely limited experiences with OS 8.6. Know nothing about 8.5.1...