16GB is pretty much all you need although it is for my case, I got about 3GB left on it and thats without downloaded games so there is still lots of space for me to use for whenever it is needed. Besides i have a lot of movies in my Memory card so thats why a lot of memory is used but i can always delete it. The problem that a lot of people have is that they have a lot of music in there so i suggest making a lot of room for games that u will play.
32GB is really not needed unless u have a ton of music or videos, if u dont have much of that then a 16GB is perfect for u!...... enough to put DEMOS, DOWNLOADED GAMES, and even APPS.