You're going to want to get a 5th gen as the sound chip is better overall than the classic provides. Cleaner sounding and more balanced. I really wish they'd upgrade the DAC on the iPod Classic though its so muffled and terrible sounding compared to the other family members. The Nano and Touch both sound very noticeably better than the Classic does.
There is one big interface drawback with the 5th gen though that drove me nuts with my 5th gen. Artists aren't displayed in songs which makes it terrible when you have multiple songs that are named the same.