The main advantage of the Duet is that you can see and navigate through your music library with the screen on the remote. With a classic, you have to be close enough to read the LCD panel on the squeezebox itself. It's lettering is pretty big but I'm obviously getting older (stupid flashing birthday cake icon) and it's now a serious squinting process to see it from the kitchen.
While I throw in another vote for the Squeezebox (my favorite indoor toy) I'll also ask if you might already own a PS3 or an XBox. They will play music from a USB connected hard drive or accept streaming music from a PC. Along with the Squeezebox in the living room, I use the PS3 in the TV room to play flac files stored on my PC using TVersity as a free music server program.
