Another great M6 recording from Vienna and the Musikverein is the Lorin Maazel one on CBS/Sony. Unlike much of the rest of his Mahler cycle, the 6th was very much 'up to tempo'. It's also a very intense performance. Depending on just how good the Sado one turns out to be, I may 'pull the trigger' on buying a Japanese edition of the Maazel/V.P.O. M6 on E-Bay, which is available at a very reasonable cost. I used to own it back in the early 1980's, and stupidly got rid of it (it came with his M5, which isn't quite as good).
I think the M6 and M8 are the two best from the Maazel/V.P.O. cycle (I'm not crazy about Kathleen Battle in the much ballyhooed M4 recording).