Simply put, don't buy a Slim if you can afford a Pro. The price of Pro is obviously more than ps4, but it's just incremental, it's not like double the price. If the extra cost is not breaking the bank, why not get the latest and greatest (and potentially more future proof)?
It is a common misconception that you need 4K TV with HDR to make full use of the ps4 pro. Let's just simplify the maths: (a) the Pro has like 2x the graphics processing power, (b) 4K resolution has 4x the pixels of 1080p.
Scenario 1: You are still using 1080p TV. Using the Pro means it has 2x the power to produce the same resolution output. Obviously this gives better frame rate and the developer can add more graphical enhancements (better lighting, better anti aliasing, higher resolution texture, better drawing distance, lock it at 60fps instead of 30fps, etc). Now this requires the dev to go back and patch existing games (the so called "Pro Enhanced" modes), not all dev will do that but going forward, new games will have that cos the Pro is already out and the dev will want to support it. And for those that the dev are not patching, Sony has added a boost mode to make them perform better (basically making unpatched games run on the Pro as if they are running on an overclocked normal ps4).
Scenario 2: You are using 4k TV. Here the Pro has 2x the power but it has to deliver 4x the pixels. Ok it's not a direct linear relationship where you need 4x the power to deliver 4x the pixels. But still you can expect that it's not always going to make it, except in very simplistic graphics games. Most games will need to use some tricks like checkerboard or dynamic resolution to deliver 4k and some games may not perform that well. That's why new games have many modes, like you can choose running 4k at 30fps or running 1080p at 60fps, or some dynamic resolution at 60fps.
If you have 4K tv, you definitely benefit from the Pro (at least games will give you various options and you can test them out, pick one that looks best on that 4k screen). If you have 1080p tv, you actually stand to benefit even more from the Pro (2x the power to deliver the same resolution, you do the math).
