The Mahler Ninth finale actually occurred to me simultaneously with the Mahler 10th: and I chose the latter for the quip (yes, I am not serious here) because it is a little more harmonically and motivically adventurous than the former, although there is a case to be made for it! The Bruckner Ninth's radicality seems just a little more in line with the Tenth's opening movement, which at least matches, if not surpasses, the drama of the Adagio in the Bruckner.
I am generally puzzled by comparisons between Mahler and Bruckner. Aside from the fact that they wrote symphonies that are very loud and go on for a long time, I see them as diametrically opposed.
Bruckner is the epitome of the German symphonic tradition, to the extent that he sometimes seems like a parody. The symphonies are based on thematic development, and are organized through essentially musical argument (contrasting and development of themes, harmony, key relationships, etc).
Mahler introduced extra-musical elements into his symphonies. They strike me as Burlesques, with explosions of sound based on his sometimes odd philosophical notions. I've just listened to Mahler's 3rd and athough he can craft music that is constructed symphonically (the finale) most of it is like a bizarre pageant.