OK so I have iPhones currently running both versions of iOS, so I can verify this:
In iOS 6 you were always presented with the slider screen. Pulling up would convert it to the button screen. In iOS 7 you are given either or.
The two screens were components of each othera single format. I'm scratching my head because I thought this was one of the most elegant additions to iOS 6.
I just tested this. I called my iPhone running iOS 6 while the phone was:
Locked: Slide to answer
Unlocked: Two buttons-- Decline and Answer