Who should get 64GB?
Apple shook things up this year by doing away with a 32GB option. The good news is, it means you get 4 times the storage for only $100 more. For most people, 64GB will be plenty of space and then some. You can store thousands of songs, lots of apps, a decent amount of photos and video, and still feel like you can breathe. If you're planning on using iCloud Drive in conjunction with your new iPhone, 64GB should be more than enough.
If you're planning on using your iPhone 6 or iPhone 6 Plus as your primary means of consuming media content, or as your primary camera and video recorder, the 64GB model will serve you well for a long time to come.