I think it's more complicated than just that. Bernstein lived for the moment from beginning to end. That approach sometimes got in the way of having an overall, "classically structured" plan for any given Mahler work. The latter Bernstein performances tended to get ever more expansive (longer), simply because he was achieving ever more power and weight in his performances. Fischer has a terrific finale that sounds fast and fiery in the quicker parts. But keeping in mind that the sixth is also very much a "classical" symphony, Fischer also treats the first three movements as though they were merely preludes to the finale - they're a tad light weight, in other words. However, as I've pointed out before, Fischer does an outstanding job of making the contrasting "Alma theme" - second subject of the first movement - jump right out of the box. Alma is young, lively, and a bit impetuous here; not just lovely and a bit sauftig. Truly, it's a case of apples and oranges. I would take the Fischer over Bernstein's Vienna remake (but only but a hair's length), but perhaps not over Lennie's earlier N.Y. one. In some respects - not all - I think the '70s one captured on video may be the best M6 from Bernstein.
Barry