I inquired to S4BB about the differences between the two apps since I did not want to spend $50 for NextAction. The reply I received was TaskMaster is more mainstream and NextAction is more robust...no specifics.
I also asked about a trial version of TaskMaster and they sent me a full unrestricted version of TaskMaster to "try". It works well, especially for "free". So far I have no need to buy NextAction.