You also have to remember one thing when you talk about a prog rock crowd.
Prog rock fans (the aging ones specifically...especially the ones who are OL savvy) are the type that do not get locked in the past. They are always pushing forward.
I was listening to thrash in 84-86, *before* it exploded in the late 80's. I was listening to college rock and grunge in the late 80's, *before* it exploded in the early 90's. In the early 90's I was listening to Dream Theater, *before* they went on to become what they are now. I found out about Porcupine Tree in 2002, now 10 years later, a few of the kids on my block are PT addicts. Now I have moved on to something else.
I voted for PT because I like them better. *NO BAND* is considered to be "godlike legends" until they have been around awhile. And with the diversified way that music is distributed nowadays, I don't see how *any* new and interesting rock bands will ever manage the sales status that Led Zeppelin achieved ever again. So you can't go by sales...even though sales is what has essentially made Led Zeppelin so legendary. What I mean is that, if the true "next Led Zeppelin" came out today, they wouldn't achieve the record sales, reputation or fan base...even though they might be just as talented, innovative and forward thinking.
I grew up with Led Zeppelin, I think they are one of the most amazing and important bands that ever existed. But in the broad scheme of things, they were an innovative blues band who was in the right place at the right time....and they are not going to get "untouchable" status from me just because of that. No one is *totally* untouchable. There will always be another band with a new and interesting and innovative sound.
I voted Porcupine Tree, because I like them better, and I think they deserve it. If we were still living in a time when you could *ONLY* get music from a record store, Porcupine Tree would far bigger than they are...and they would be considered *rock legends* in about a decade from now. But that's another down side of the "two edged sword" that is the new age of music consumption.