Iris

Pretty much nothing. Some games will look slightly sharper on Pro if you have a 4K TV and some will run at a higher frame rate. But honestly, it's not a huge difference.

It'll be worth it if you 1) have a 4K TV, or 2) would like to play your games with a higher* framerate (the smoothness in games will be better). I highly doubt that there will not be a PS4 Pro exclusive game, even though Sony said everything on the Pro can be played on Vanilla, but you might want to consider it as well if a game comes out later that plays like crap on PS4 but okay on PS4P

Tl;dr if you don't care about FPS at 24+, get the PS4. Otherwise PS4 Pro.