Definitely go for a 64GB!

I have a 16GB black one and I got seriously dissappointed when I tried to move some of my music library to the phone. The point is that you cannot access all of your memory for your own use. The system partitions it into several (for the moment) unchangeable and mostly inaccessible pieces. Therefore there are less than 10GB for music, movies, documents and pictures as far as I remember.