I don't think graphic novels are as popular to manga/comic book readers. For one, they tend to cost a little more (usually around $25) & you'd probably have a little difficulty find all but the most 'popular' titles in graphic novel format at your local comic shop. The good thing I CAN say about graphic novels is they usually are compilations of a complete volume of a title & in my opinion, worth the extra $ if you don't wanna dish out that $15 or so bucks per one issue of that same title.
But hey. All my synapses don't fire, so what do I know?