The shuffle sucks. I mean, it's great if you spend 45 minutes a day on a treadmill and just want some noise filtered into your head while you do it. But, there is no screen and no way to play stuff in order... it's absolutely horrendous if you use it for audiobooks or podcasts.
Basically, as far as I understand it, you have the Shuffle, Nano, Touch and Classic. The Shuffle, as mentioned above, sucks ass unless it's used in a certain way. The Nano is a smaller version of the Classic both in hard drive size and physical dimensions. The Touch is an iPhone without a mic or speaker... like, it is literally an iPhone. It's got a slightly larger drive than the Nano as well.
The Classic is my personal favorite, but that is only because it's got the largest hard drive. It's just an updated version of the original iPod and is about the same size as the Touch... maybe a bit smaller. It's a breeze to carry around, holds roughly 29,000 songs and I would be completely and utterly lost without mine.
If I were you, I would focus on the Nano. It's really tiny and they have a decent enough sized drive in them to hold quite a bit of stuff. It's also got a color screen for browsing and most if not all of the functionality of the Classic versions.