A R7 1700 is $100 cheaper than a 8700k on Newegg.ca. And can be OC'd on a Hyper 212 cooler easily.
On a $120 B350 mobo.
8700k needs not just an AIO, but a high end one to OC. On a $200 plus z370 board.
And runs very hot even on a 280 mm hi end AIO when OC'd aggressively.
Meaning, for me at least, if I was making this decision, the platform cost difference is $260 ish CDN.
And you can actually buy a 1700. Which is on a platform that will be supported for several years.
If you wanted to go from an 8700k to the not yet released consumer 8 core, that will need a Z390 board.
8700k is a very good cpu, "better" than the R7 series. But a better deal it is definitely not.
I would be taking very careful look at all the factors if faced with this purchasing choice.
Since the 8700k is highly likely to be overpriced and/or sold out for the next few months, don't be in a hurry here.
Your Mileage May Vary