They're just a metal band that writes long songs. And as I said in another thread, unless I'm missing someone crucial, prog is not progressive.
IMO, they are the same thing. The distinction is just a one made up by people who take music really seriously and want to distinguish bands who make music in the styles of the 70's prog rockers (prog) with bands who genuinely try and push the limits of music in their own way (progressive).
I have no problem calling "prog" bands "progressive" bands, and "proggressive" bands "prog". From my perspective, bands who follow the tradition of 70s rock can always be referred to as progressive bands OR prog for shorthand, whereas the more modern genre-defyers are generally tacked with the label "experimental" instead.
No matter how you slice it, DT are both progressive AND experimental, though they were more balanced in the early days, whereas now I think they pretty much stick to their own confines (which is fine). I don't mind DT sticking to prog, but I would like a little bit more of that early genre experimenation/fusion return, like the carribean sounding music in Learning to Live or the Jazz across SFAM.