Well, it so happens that I've got both an iPod and a Riot sitting on my desk as I type. The Riot has more storage space (20Gb versus the iPod's 10Gb), but the iPod is smaller -- not by as much as you'd think, though -- see the attached.

I can't comment on the UI -- I've not used the Riot that much, but the iPod was initially a little odd, when compared to the car player (for example).

Personally, I think that the iPod is cuter.