With your Sony can you control what music it syncs with on your music manager? With itunes you can either make a playlist or check/uncheck tracks and regulate what the ipod syncs with. So you can have a music library which is much bigger than the ipod's memory. Can you do the same with the Sony? 20gb is big enough that you can use it as if it were a CD multichanger, you just need to remember to switch about what you want to listen to.
BTW the ipod Classic sounds brilliant and reviews continually put it at the top of the pile. I have not heard the Sony.