[Steatopygous]

Excellent recommendations above. But for me, the most important Mahler 9 is the Bruno Walter with the Vienna Phil from 1938. There is a tension yet resignation that is entirely appropriate - six weeks later Hitler annexed Austria and Walter had fled, along with (eventually) many Jewish musicians. (Some who did not died in concentration camps.)

 

[Steatopygous]

I concur with Skil's sentiment; I vote neither and suggest Rattle (BPO), Abbado (BPO), or even Chailly (Concertgebouw). Or Tennstedt, obviously, with LPO, which is always a fall-back option (read: absurdly high baseline). Karajan not on my radar. Not a fan of his Mahler stuff.

Love your avatar. When a relationship ended in 1977, the only thing we fought over was who got the Tintin books. I did; she got Asterix. She got the dinner set; I got the car (she got the higher value item).

 

[Steatopygous]

That's a fascinating experiment. How curious that Walter is so fast, and Klemperer is also relatively fast, despite his reputation for slow tempi. The point being that both of them knew Mahler and studied with him.
I'm not suggesting there is only one right way, but it does seem that Mahler might have favoured crisp tempi. Of course, he never conducted this work.