It depends on how much music you have. If you have under 5 gigs get the 5 gigger. If over, get the 10. I'd advise against the 20, simply because it's bigger, and it can hold more music than anyone could know what to do with. I'd say go for the 2nd Gen 10 gigger with the touchwheel scrollpad.
That is, of course, if you really want used. As for real value, you'd be hard pressed to find a better value than the current 15 gig model. It's just the perfect iPod in every way.