Here at NoveList, we have debated this question for years.
Finally, we created these genre headings:
GX "superhero stories"
(for prose fiction)
GX "superhero comics"
(for graphic novels)
Don't feel like searching?
Check out our Recommended Reads list,
Superheroes for Adults (UI 428957)
If you work with young adults, check out:
Superhero Comics for Teens: Where Do I Start? (UI 440372)