I've got a 64MB N9, and frankly, I never seem to have more than 5-6GB on it, even when I throw some videos and maps on it before travelling. You're probably fine with the 16GB unit.

Another tip about music --- if you spend most of your time around a WiFi network, Spotify is available from the Ovi store and works great. My N9 is almost always streaming from Spotify, making storing MP3s on it a non-issue.