It's fairly simple.
Assuming you are in the US, and you can afford $500, get the 64GB.
If you can only afford $400, get the 16GB. If you find that it's not enough, return it, save up till you reach $500, and get 64GB.
The beauty in the US is there's a return period. You don't have to feel so bad picking the wrong size early on. So I would just pick 16GB and see if you can live with it.
Assuming you are in the US, and you can afford $500, get the 64GB.
If you can only afford $400, get the 16GB. If you find that it's not enough, return it, save up till you reach $500, and get 64GB.
The beauty in the US is there's a return period. You don't have to feel so bad picking the wrong size early on. So I would just pick 16GB and see if you can live with it.