Haitink has been an awesome Mahler interpreter in the past, and one which had a great role in bringing Mahler to the masses. However, recents things I've heard from him failed to particularly impress me (but I have to say that I don't know his 9th). It looks like his Mahler hasn't improved with the time (it has become sloppier in many respects), something which can't be said about some other conductors (Abbado comes to mind)...
As for me, I think my first ever live M9 has been here where I live (Lugano, Switzerland), that was with Daniele Gatti conducting the Royal Philharmonic. Then I heard Chailly at Milan with his Verdi italian orchestra (wasn't bad, but ofc the Concergebouw would have been another history), Rattle with the Berliners at Lucerne (wasn't moved at all, and that applied to a good part of the public as well, something almost unbelievable given the emotional impact this symphony should have, almost independently from how it's played...the playing was pretty much immaculate, but the soul just wasn't there), Abbado at Lucerne (twice) and again in Paris (no words can adequately describe those concerts), and a couple of weeks ago again in Milan with the Verdi orchestra under Axelrod (a very expressionistic view, I'm not a fan of this way to do it, but it wsn't bad at all).