I agree both companies have had major comic characters and I would say that it is a tie.
However, basing on artist & writer talents, I think Marvel has the edge. They have had everyone from Jim Lee, Todd McFarlane, Rob Liefeld, arguably the most prominant artists in silver age history...even golden age, Jack Kirby. I was really into comics around the fruitation of Image Comics, but haven't really kept in touch with it in nearly 10years. I just went to buy a few issues of Superman drawn by Jim Lee not too long ago though.