While I still did not 100% decide on this and will at least wait until the Pro is available including reviews... I heavily lean towards the PS4 slim.
Now some background... I do not have any PS4 system yet. I have comfortably settled to WiiU, PC and handhelds (PS Vita, 3DS) the last few years (and lots of retro games available on those systems), holding back on the purchase of a PS4 because I wanted to see what the 'Neo' is all about. I also have no intention to upgrade my TV set to 4K yet, the jump to HDTV was the one and only time I've ever replaced a TV without the old device being broken or unusable. I am very sensitive towards "upgrading costs" if they can be avoided (e.g. I also do not partake in the smartphone upgrade game) and energy efficiency and system stability (meaning I prefer second or optimised revisions of devices). Also, while I am interested in it, I don't think I will get myself a VR set within the next 1-2 years.
That said, I also would in general like to get the best possible experience. Some of those best experiences, I can get on PC. I like several of the PS4 exclusives and play a lot of Japanese games, which is why I still want a PS4 at some point (e.g. Gravity Rush 2, Valkyria Chronicles, King of Fighters 14, Persona 5, Yakuza 0/6, Nioh, Nier, Musou games and so on). But - most of my interests are more within niche genres or franchises which likely won't profit -as much- from a better PS4 without a 4K TV. Adding to this, the PS4 Slim sits at half the energy consumption of the PS4 Pro (saving energy/money in the long run) and seems to be very quiet. I'd probably rather get a good deal for the PS4 slim and use the saved money to invest in a couple of more PS4 games or put it towards the Nintendo NX.
But as I said in the beginning, I am still waiting for the PS4 pro to come out and read impressions/reviews on that. I've waited long enough for a potential PS4 purchase, I can wait a bit longer.