I think it is harder to have more complex tunes stuck in your head. I will sometimes recall fondly shotakovich (symphony 11, movement 3 usually) in my head for about a half hour or so, but that is nothing compared to the unrelenting mind-invasion that old-time (traditional appalachian folk) tunes have.

Regardless of the individual variance in the complexity of the "earworm", one of the very interesting things is this interplay between short-term auditory memory and the loop being replayed. It will be even more interesting to see if their are ways to exploit this to effectively keep more things in working memory.