Sabbath and Zeppelin

I've always considered Black Sabbath to be quite progressive songs, in fact the album Sabotage is pretty progressive as a whole with songs like Symptom of the Universe (also a great metal song), Megalomania, Supertzar and The Writ.  Other progressive songs in their repertoire include War Pigs, Behind the Wall of Sleep suite, A Bit of Finger suite, Hand of Doom, Wheels of Confusion, Killing Yourself to Live, A National Acrobat, Gypsy, Dirty Women, Air Dance, Heaven and Hell, Sign of the Southern Cross, among others.

Led Zepplin have also had many progressive moments, Stairway(obvious), Achilles Last Stand (great intros and outros), In My Time of Dying(in fact most of Physical Graffiti), No Quarter, In The Light, Carouselambra, but I've always associated Led Zepplin as more of a classic rock/blues/folk band. I don't know... definitely progressive moments but thats just the feeling I get when I think of Zeppelin.  I suppose the same can be said for Sabbath, in the sense that they are just a hard-rock/blues/metal band...

But I would support either band since I believe they both have a significant amount of progressive material.   

That's quite simple: it's because "progressive" is a vague, highly subjective notion, and because different people will react differently to a given piece of art.

Art and language are open to interpretation. Until we all agree as to just what "progressive" is (and we never will), it is pointless to say "this is pretty progressive," or "that song is progressive" as if you were offering convincing evidence, or stating verifiable fact. All such statements are mere unsubstantiated opinion, and the "discussion" or "argument" merely becomes a pointless exercise in contradiction.