This is a topic that is bugging me for awhile now and I will just write my personal view and opinions. First let's break it down, as every good Prog Rocker know, Progressive Rock is a musical movement that started in the late 60's and emerged/flourished in the 70's, in my personal opinion is the most awesome moment in the history of rock and music in general, there was this flow of creativity which, in my case, I truly love. In that period bands like, Genesis, ELP, Gentle Giant, Van Der Graff Generator, YES, Camel, King Crimson, Jethro Tull etc. released wonderful albums and music, just a bomb of creativity exploded, unique music and it was wonderful, this musicians making this albums were more of a jazzy and classical background, I mean, they did rock music but inspired a lot in the jazzy and classical compositional templates, that are very free in terms of what you can do. The bad thing is they didn't get popularity, it was a very crazy music for the masses, it was very segmented in a way though.
Now, in our era, or in my era to be more precise, which is the 90's and the present time, bands like Spock's Beard, Porcupine Tree, Dream Theater, The Flower Kings, Transatlantic, were inspired by the 70's, but there's a trend that they are NOT truly the progressive rock of our age, that they are just "copying" the bands or progressive movements of the 70's and just writing music in that template and not making something "unique". However, is true that there are much similarity in that sound, but, is just influences! Some artists and musicians friends alike cite that the true progressive rock of our era and the true "music of the future" is bands like Radiohead, Mars Volta and Tool, don't get me wrong, these bands are great, sound modern and are truly unique, yes, UNIQUE, and that's important.
But, they lack that "essence" of musicality maybe, I mean, is wonderful music, very unique and full of experimentation, but is not that musical compared to the other kind of "modern" prog that is available now, is music more focused and done by instinct rather that true musicianship, and that's one of the definitions I have for Progressive Music, is music with high musicianship. So, now progressive rock or progressive music, it is divided in two worlds; The one that is "modern" and the one who sound more of the music of the 70's era. So let's put up a name to them for a better understanding now: Post-Prog (Radiohead, Tool, Mars Volta), and Neo-Prog (DT, Transatlantic, Flower Kings, The Tangent) Progressive Metal which is another great and popular kind of progressive music, will be included on the Neo thing, just to divide both worlds.
Both worlds are very different, and in fact, I don't think that Post-Prog has any kind of influence of the great prog bands of the 70's, it's just rock music with a lot of experimentation, and creative production, that's how it emerged I think... Neo-Prog in the other side tends to sound more Symphonic, and Eclectic, mixing jazzy elements and stuff, which for me have a greater value, I mean, some people see this kind of world as a very "pompous" one, but it's not, is just music with more musicianship, is music written by musicians, in the other hand Post-Rock it is NOT in that kind of stream, there are just a bunch of guys with great talent who picks up an instrument in their garage and experiment a lot making music, and the result is that.
I know that there has been a great arguing in all this, and it is quite a complicated topic to discuss, but I have my personal preference and taste and I go better with the Neo one, is more MUSICAL, and I'm a musician trained in harmony, trained in songwriting, trained in my instrument, and that have to be tested on music, and the music for that is that Neo-Prog thing.
Post-Prog, is more mainstream, is true, is more accessible to people, is very influenced on Space Rock and Ambient stuff, which is very cool, but you know, is very easy to write, very easy to perform... take Porcupine Tree for example, it is a great mix of Pink Floyd, Ambient and Tool, Space Rock emerged with Pink Floyd, and believe me Pink Floyd was a very, very popular band, in the package of prog band of the 70's Pink Floyd was without a doubt the more accessible and popular, so of course it will get more mainstream view, and that in a way is good of course! don't get me wrong, ambient stuff and space rock can be a very easy music to write and perform but those elements can add that touch of eclectic-ness to a band sound!
Conclusion, there are really two worlds and both of them have pros and cons, in my case I could release tomorrow an album with rock music mixed with ambient stuff and make a great mainstream "progressive" band, but I want more than that, I'd spent lot of time with music, writing it, and with my guitar on my leap, so why just make music with ambient and noise? it's not the case for me, the thing for me is that I have to feel challenged every day, if not, why do it? so, I truly don't know if my future band would fit in one of those two worlds, of maybe the two at the same time, I really don't know, the worst thing to really do is to be predestined.