The Busoni has it beat, both in length and in quality, in my opinion.
Well that is certainly my opinion also, though I've always had a very profound love of the Busoni, so perhaps I'm biased. Yet the percentage of folks here with experience with
both the Busoni and Furtwängler may be rather low. No value judgment meant, btw, just 'experience' as I mentioned.
I thought the concerto didn't go anywhere in all of it's massive length, and I feel the same way about Furtwängler's symphonies. He should have just stuck to conducting. Then again, I am not much of a Bruckner fan.
I've heard the Furtwängler 2nd Symphony several times, it is critically considered his best (make of that what you will), but it's really hard to get away from the very apparent fact that Furtwängler simply lacked the ability to control long spans of time. Compare it to other expansive late romantic symphonies of, say, Bruckner, Mahler or Elgar and what is missing from Furtwängler's technique becomes quite obvious.
The ultimate master, IMHO, of controlling large spans of time is Wagner, and Furtwängler was -and still is- one of Wagner's greatest interpreters. Furtwängler simply couldn't apply what he instinctively understood about Wagner to his own compositions.
Sorry that you don't care much for Bruckner. I could comfortably just tell you that Bruckner is an
aquired taste, but I'm not so sure about that. It's not like coffee or some brands of beer. No, like certain composers, I think you either respond to Bruckner or not. A very good friend of mine -old enough to be my grandfather- is a lot more experienced and astute musician than I, but he's always hated Bruckner.