from reading the reviews on amazon i made a list of pros and cons
zen vision:M
pros: higher res. screen, better battery life, customizable background, comes with ac adapter
cons: bigger, $30 more than ipod, not as many useless accessories
ipod video:
pros: you'll be like all the cool rich people
cons: lower res. screen, less battery life, non-customizable background

edit: i forgot, the vision:m also has fm radio/fm radio recorder