The TomTom is techically a much better unit than the Zumo, the others dont really come into the equation due to cost vs feature.
Dont buy a TomTom Rider V1 if you can help it as they have some issues with the old mounts, the new one is perfect.
With TomTom you get FREE map updates from their mapshare system, itinerary planning on the device, not off the device like the zumo, and its SOOOOOOOO easy to use its a joke.
There are no ports on the TomTom, only on the Zumo which has proven to be one of the Zumos flaws. Everything audio or traffic etc related is done over bluetooth with the TomTom.
The only thing the Zumo has that the TomTom doesnt is MP3, incidentally MP3 can be added to the TomTom by buying some third party software.
If you were riding in North Africa or India etc, then I would get the Zumo as you have the option of getting maps for the middle of nowhere, if your riding anywhere in europe, america, australia, and a good many more then get the TomTom.