Just go with the 16gb phone. I might have misunderstood, but you sound just like me. I'm not interested to have all my music and movies on the phone. I just copy the videos on the phone that I expect to watch during the next few days or weeks, and the music that's been on my listening rotation. 16gb has been plenty for how I use the phone.

But since there is no way to extend the available space, you should definitely think how much data you want to store in it. From your description it sounds like 16gb will serve you just fine.