And, more than any other symphony, getting the opening movement correct is key. Normally, I have a very good understanding within 2-3 minutes of just how good, or bad, a first movement in general, and likely a whole M6 in general, will be.
Too many conductors pay too much attention to the "Ma non troppo" and not enough to the "Allergo energico" notation of the first movement. Among those that do pay attention, half or more of them don't nuance the tempo enough (listening to Bernstein now, with just that problem). And, for a few who get the whole first movement more than halfway correct on "snappy but nuanced" on tempo, and on good volume, usually either get one or the other of the middle movements a bit wrong, or get the finale way wrong.
That said, in an M6, I prefer the Scherzo come before the Andante on the middle movements. And in this structure, especially, it's key not to dawdle the finale.
I had new hopes in listening to Zinman's M6, part of a new recording release of the full Mahler cycle. He's better than Boulez on this one, in some ways. However, he clocks the overall symphony at more than 80 minutes, meaning he's dragging somewhere. Ditto on the M2. (Note: Every Mahler symphony, except the 3rd with repeats included, should be a 1-CD affair. And righly so, without rushing.)
And, not, this isn't "just me" saying this, and even if it were, it's not because I'm cheap about buying CDs.
Reportedly, the composer himself, baton in hand, clocked the 7th at under 75 minutes at its premiere.
Due to this, primarily, a lot of modern conductors whom many classical aficionados tout as being great on Mahler are bleah or worse in my book.
Michael Tilson Thomas is so-so at best; semi-hit and more-often miss might be more accurate.
Simon Rattle is simply horrible. I heard him, and saw him, on PBS, not too long after he got the baton in Berlin, do either the 2 or the 6. It was awful.
Bernstein? Lenny was OK at times, godawful at others. I heard him earlier doing a Mahler 6 at Vienna. Certainly "allegro energico," but certainly also totally lacking in tempo nuance, per my plaint above. (Of course, Lenny's Beethoven Ninth after the fall of the Berlin Wall showed just how much he lacked nuance. The finale was the worst Beethoven train wreck I have ever heard.)
Giuseppi Sinopoli got touted for his alleged psychological insights in the early 1990s, when he started branching out into non-operating major conducting. His M6 got draggier with each movement; the finale sounded like a funeral march and I've never bought a CD of his stuff since.
Gustavo Dudamel knocked the socks off the Shostakovich 10, or at least the "Stalin's dead" movement. But his Mahler is almost somnambulent.
As I noted above, Boulez is generally good on Mahler tempos, including some degree of nuance, though he misses some of the late-Romantic side of Mahler. But, he's not always great, or even very good. His M2 just is wrong, and that's with having recorded it before officially jumping into the full Mahler cycle with Deutsche Gramophone, then again as part of that cycle, near the end. He blew it both times and didn't seem to have learned a lot in between.