My music library is around 15GB, but I don't listen to all of it all the time so I went with the 16. I put my favorite stuff on the device, then I stream the other stuff using Orb or Pandora if I want something new.
I currently have 4GB free after taking some pictures and video at a wedding. I typically take my photos and videos off my camera after taking them so they don't really end up using permanent space on my phone.
I also have the whole country Navigon app which takes up 2GB on its own.
Really it just depends on how much stuff you want to have with you at any given time and how often you sync your phone to your computer.
I currently have 4GB free after taking some pictures and video at a wedding. I typically take my photos and videos off my camera after taking them so they don't really end up using permanent space on my phone.
I also have the whole country Navigon app which takes up 2GB on its own.
Really it just depends on how much stuff you want to have with you at any given time and how often you sync your phone to your computer.