Let me get this out before I lose my mind. If you already have a PS4, and are considering a PS4 slim, don't. The PS4 Slim is utterly useless if you already have a current PS4. It also lacks a very important feature the original PS4 has, an Optical port. I for one use my optical port all the time either with my Marshall speaker or my Lucid LS30 headset. Now, if your on a budget, don't care for 4K, don't care for an optical port, then the PS4 slim is your console if you want a Playstation. If you really want a cheaper PS4, buy an original PS4 when the PS4 Slim is released. The PS4 Slim will drive the price down of the original PS4.
The PS4 Pro is another matter all together. Coming in at $399 and packing a significant power increase, it is sort of a mid generation game changer. It will be able to game at 4k(probably upscaled, not native), stream 4K content, and utilize HDR. In actuality, Sony announced they will be pushing a firmware update so all PS4's can utilize HDR in some way, not just 4K TV's, but normal HDTV's as well. I have no idea how that's gonna work, but it will be great if they did.
Back to the topic at hand, the PS4 Pro is the system to buy if your wanting a PS4 and don't have one already, and for those enthusiasts who want to take advantage of the most powerful Sony console has to offer. I 100% recommend buying the PS4 Pro over the PS4 Slim...
There is just one tiny issue the PS4 Pro has that may be of concern though. The PS4 Pro has no 4K UHD player. So no watching or owning the current UHD Blu-Ray's on the PS4 Pro. Want to watch 4k content with HDR? Watch Netflix or some other streaming site that offers it. Or buy the Xbox One S for $299 and not only do you get a 4K UHD Player, you get 4k streaming, 4k upscaling of games, and HDR. This pretty much checks all the boxes for someone who's wanting to take advantage of all the 4K content coming out this year and beyond.
So here's the skinny of it. Want a PS4 and don't care for 4K content, get the original PS4, not the PS4 Slim(No optical drive). Want a PS4 with some 4k capabilites packing a bit more power than the original, the get the PS4 Pro. If you want a console with no real compromises right now with a 4k UHD player, 4k streaming capabilities, 4k upscaling, built in HDR, and an ever growing library games including games from the last generation continually being added to a list every month, get the Xbox One S.
Now here's one last bit of advice for the masses. Buy the console that has the games YOU want to play.