I don't understand the difference and IMO, they are really the same thing. The only difference is that doorway pages are usually hosted on a single domain, while spun articles are submitted to the various article directories.
I'm not making any moral judgment other than how the two seem to be treated differently in IM circles, but they are really the exact same thing.
Doorway pages take the same content, make copies of the pages, then substitute keywords from page to page. Spun articles merely substitute different words and phrases, but the article still contains the exact same INFORMATION as all the other spun pages.