The nano has no moving parts - it's all flash memory (no skipping/more durable) whereas the Ipod video uses an internal harddrive. Rumour has it that the video ipod is about to be deprecated in 2007 with the release of another generation of ipod video.

The nano is obviously also smaller than the ipod video. That's all I can think of. If you're going to be taking it on rides, i'd get the nano.
Calling somnambulant...

Edit: If they are the same price the video ipod is tempting