I would be surprised if there was a difference. My understanding is that Jim wants everyone to be able to "test" all aspects of the "simulation". Registering "only" adds the number of fish one can have and... removes the need to type in test fish.... I think
Not that it's vitally important, but for completeness it would be nice if the readme.txt said what the "unregistered" limits on the fish are. Did you happen to observe that?