I'd go with 9.0/9.1. the rumor mill has been saying that "9.1" will be the last of the development for the "Classic" interface. This would be handy to have becasue this should close almost all the bugs, and rumors been saying that it has a new "Nanokernel" that speeds up the system a lot. I can't say how the new system does RAM wise, I haven't ran it.
9.0 does use more RAM then 8.6 does. So if you're on an older machine with not a lot of RAM (<64 MB, at least 128 is best now) then stick with 8.6
So if you're looking for a Classic System, I'd go with 9.0. If you're able to run OS X, then that's the system I'd be looking at.