Re: What capasity of iPhone should I get?
As kch50428 and kataran said, getting an iPhone with more space will have a better re-sale value, but it's all about getting what's best for you without over or under spending. Also, if you will not be selling your iPhone in the near future or have an issue with re-sale value, pick what's best for you.
Since your music seems to be the item that takes up most space, focus it around that and what extra you will have left. Remember that you do not get the "actual" amount of usable GB's that are stated. 16GB = 13-14 / 32GB = 57 / 64GB = 57.2 (even though mine got 55).
Always focus on what you're using the phone for and which will take up the most stuff (Apps, Pictures, Photos, etc.) and decide from there.
I will use me as an example:
I always buy the 64GB version because my music library is over 32GB and keeps growing. I also do take alot of photos and have a decent amount of apps. Even though I only take up half of the phone's storage, I still make sure to sync my photos so they don't take up too much. It gives me piece of mind knowing that I have so much space and do not have to worry about running into an issue.
Good luck! I hope you get the best storage space for your buck! (also remember that there is iCloud and other various storage apps).