Well, here is my stab at it....
I own an 83 250r. Hands down you get much better performance all around on the 85. The liquid cooled makes it much better in the summer months in regards to cooling. I know from experience :x that the 85 will beat the 83/84 models. As for reliability, hey they are Honda's, they run forever. I know it is a little easier to rebuild the air cooled r though, less parts. I think if you have the choice, spend a little more and get the 85. I love my 83r though, it's fun and fast.
Scott
400ex, looking for a 1984 250r(running or roller) or a 200x.