There are some interesting discussions of the differences/benefits of each on the Personal Brain forum at www.thebrain.com.
Also, note that David Allen will be the guest on a Personal Brain webinar regarding Personal Brain and GTD on June 3rd:
I've used, and continue to use both. Personal Brain serves as a repository for all the information in my life. Some of that information is in mind maps. I use Mind Manager as a tool for thinking, and Personal Brain as a tool for storing and accessing my thinking. You can do the same thing with multiple linked Mind Manager maps, but Personal Brain does it far more elegantly.
That said, there are those who view Personal Brain as a better mindmapping tool. However, it doesn't adhere to traditional mindmapping rules, and I find Mind Manager better for brainstroming as a result.
Check out the Personal Brain forum and the webinar, and make up your own mind. Note that the webinars are all archived after a few days if you can't make it live: