It's really dependent on the type of convention you are planning on attending.
If you're going to a bigger, more commercial convention such as Wondercon, San Diego Comic Con, Emerald City Comic Con, then you might want to try selling more fan art. Bigger conventions usually are comprised of fans of already existing properties (i.e. Marvel Comics, MLP, etc). It's just what sells better.
But for smaller events, like a small press expo, you've got a great audience that is looking for content that isn't based on existing IP.
I was at Alternative Press Expo in San Francisco the other weekend and about 95% of the convention was all creator owned work - I saw very little fan art and sales were great at that show.