The biggest difference between Dragon Naturally Speaking and Nuance lies in the voice recognition. With Dragon, you just have to speak the sentence that is being translated, and it will automatically recognizes your voice and play back the appropriate accent. Nuance on the other hand requires you to first insert a clip of your own voice into the program in order to have it recognize your voice. This has its own benefits and drawbacks, the biggest of which is that you can never be exactly like the voice that was inserted into the software, which means that your accent may vary from what you actually speak. There are ways around this however, by using a microphone and speaking through it (there is a demonstration video included with the program that demonstrates this).
Another big difference between Dragon NaturallySpeaking and other voice recognition programs is that it comes with three bonus features: dictation recording, voice activation, and goldman sachs. dictation recording is extremely helpful if you are giving a presentation at a school or university. With the recording, you can be sure that every speaker who hears your speech will understand and remember what you are trying to say. Voice activation is helpful if you need to give a speech to a class full of students, since they won't be able to get the full idea unless you say so explicitly during the demonstration video.