When I was looking into various formats for an "on-bike" set-up, the arguements seemed to be that mini-dv is a well proven and relatively vibration-resistant format. DVD seems to have the convenience of taking your media straight from your camera to your DVD player, but first gen DVD cameras are very sensitive , so are a complete no-no for on-bike mounting.
Also, the convenience issue is reduced when you consider you are almost certainly going to want to edit what you have filmed (all those bast@rds overtaking you for example)
As with all technology, its always evolving, but (particularly when considering buying a bargain "old-model") mini-dv seemed to be the best option