I have submitted many books to both companies over the last several years. In my opinion, they both seem to do a great job grading books. If anything, they tend to be overly picky. That is probably a good thing.
The fee charged by CBCS is a bit less than charged by CGC. However, I have experienced longer turnover times at CBCS. I've heard people say that CGC is the undisputed leader in the grading sector, but I have personally not observed a discernible difference in the quality of grading between the two companies. Consider the two comics below, for example, which show the same comic graded by both companies. They both look like 9.8's to me.
I usually see a book in a CGC case selling for a small premium over the same book in a CBCS case. Why the disparity? What have you experienced? Please share your comments on this subject!