Store Core Data Sqlite in NSLibraryDirectory or NSApplicationSupportDirectory?

What is the difference between NSLibraryDirectory and NSApplicationSupportDirectory? Which one is the best place to store the Core Data .sqlite file if I am planning on using a syncing service?

I currently store my .sqlite file in NSLibraryDirectory. If NSApplicationSupportDirectory turns out to be the better choice, how will changing this affect my current users? Can it easily be changed, and what are the steps I should take to ensure a seamless transition for my existing users?