I have learned over the years in general, that as I came to categorize a given work in classical music as a "favorite" through repeated listenings of the same and differing recordings, I have come to accept that there are recordings that: 1) sound great, but are inferior in performance; 2) sound inferior, but are great performances; 3) sound great and are great in performance; and 4) sound inferior, and are inferior in performance.
Many veterans of classical music who have listened for years of performances of say, M2, may state in general, that as the years have passed, recorded performances of M2 have gravitated from no. 2 to no. 1. Yes, we no longer have Bernstein, Walter, Klemperer, to name three, who most feel excelled in performances of M2, but whose recordings do not match anything in sound quality of what we have today. Some concert-going people even say that today, with the playing and performing abilities of orchestral players approaching parity in many venues, plus the fact that many conductors' academic preparation for their profession is very similar and that they travel consistently world-wide, a performance of M2 by a smaller city's orchestra and chorus might be as good, or even better, on a given night, than by one of the major orchestras in one of the world's great cities.
So what is it I'm trying tell you? It is that if you are patient enough, you can, over years of consistent, thoughtful, and open-minded listening to enough recordings of a work such as M2, you can learn this work fully enough to the point that you will be able to tell what recordings of it are good or poor in performance, and good or poor in sound.
Bottom line, do not let sound quality be your sole criterium for what you obtain in Mahler recordings. Give consideration to what are considered GREAT performances, even if the sound is dated. If you have access to a library that has recordings you can borrow for free, take advantage of it. If you have classical music friends who share your enthusiasm of Mahler and are willing to loan you their Mahler recordings, take advantage of that as well. There are Internet downloads; many are free or charge a small fee. Through your various networking contacts, you will, through years of listening experience, be able to tell for yourself what Mahler recordings not only sound good, but are great performances.