I guess we could say that Bernstein had three distinct periods of Mahler interpretation. By "interpretations", I basically mean tempo relationships, as matters of texture, phrasing, balances, and local effects, essentially remained consistent (based on doing what the scores say, as Dave points out). The early period - the 60's - is perhaps that one that all of us are most familiar with. They featured the N.Y. Phil., and tempi tended to be on the fast side (allegedly, Bernstein wanted to the M9/IV slower, but was encouraged not to because of the constraints of LP sides). For my money, this still remains his best overall period.
Then came the 70's, obviously, and Bernstein's big break from the N.Y. Phil. While it may be an exaggeration that Bernstein taught
Mahler to the Vienna musicians, they certainly received a much bigger dose of it than they had in previous decades. More important, he brought Mahler home, so to speak, to the Viennese audience. Now Mahler is every bit as much standard Austro/German fare in Vienna as Beethoven or Brahms. Some folks feel that Bernstein's 70's cycle, captured on video by Unitel, was his best period from a purely conducting standpoint - combining that which is best about his early and late Mahler. To some extent, I agree with this idea. However, the Vienna M8, great as it truly is, is remarkably similar to his earlier London one from a purely conceptual basis.
Bernstein's 80's Mahler employed slower tempi (M4 is an obvious exception), but also exposed a certain deepening of his interpretative profile. Perhaps his Amsterdam M9 is the most radical example of late Bernstein, as his Rondo-Burlesque was fast and furious, only to be followed by one of the slowest and most introspective adagios (4th movement) imaginable. Bernstein had been penciled in to do an M8 in Amsterdam. I, for one, am sorry that that never materialized. But, we have two generally outstanding M8's from Bernstein as it is. That, my friends, is where I'll leave my zwei groschen regarding Bernstein's Mahler. I have no regrets that he didn't take up the M10 cause, as we have plenty of folks doing that already.
Barry