Chris Heard, an Associate Professor of Religion and Blogger at Higgaion, has written a pretty extensive product piece comparing MindManager 7 Mac with NovaMind 4. Chris believes "the improvements in MindManager 7 outstrip those in NovaMind 4. MindManager 7 is easier to use and more Mac-like than NovaMind 4."
The blog posting includes some head-on comparisons between MindManager and NovaMind’s user interfaces, user friendliness, features/capabilities. In conclusion, Chris offers a final thought: "MindManager 7 has so many advantages over NovaMind 4 Pro that it will almost certainly become my mind mapping software of choice."
You can read the full product comparison in more detail here.