Cant answer this for the S3, but for the iPhone I would always use an anti-glare screen protector along with the built-in otterbox screen protector. If you didn't, the otterbox's screen protector would stick to some points of the screen (not all) and make it look like water was trapped under the screen protector.
I am anxious for the size specs of the S3 defender case to see if it is going to fit in my adjustable proclip case that I use right now.