iCloud Photo Library vs My Photo Stream Now that the iPhoto iCloud Library feature is available on both iOS and OS X devices, what is the purpose of keeping the Upload to My Photo Stream feature?

If I understand correctly, with iCloud Photo Library, every time I take a picture the picture goes to iCloud and is pushed back to all my devices. 

What are the basic differences between the two services? Why should someone use one versus the other one? Why should someone use only one of them?
iCloud Photo Library uploads every photo on every iDevice to iCloud and syncs then. My Photo Stream only saves the 1,000 most recent photos. Basically, the only advantage of My Photo Stream is that it doesn't count against your iCloud storage, whereas iCloud Photo Library does.