If you store your photos and music the right way 64gb is more than enough! I would struggle with a 16gb with videos and pictures alone.
Had 32GB 5S now with 64GB 6 Plus and running low on space. Never thought I'd use up so much so quickly. Unlike most, it seems 128GB might have been the right one for me, even though there would have been a lot of excess space with the 128GB.
What's the ideal or 'right way' to store photos and music?
Maybe use a streaming service or iTunes Match. Then maybe use iCloud photos or some upload service like Dropbox or onedrive. Keeps the photos in the cloud but not the phone.
Honestly, 64GB is the sweet spot unless you're just a hoarder of multimedia. I can't even fathom using more than 64GB unless I stock my phone with all sorts of movies and music I rarely ever watch or listen to.