The 7700k will get better fps than the r7 1700 at 1080p. There are many reviews to support this information. You can read the review on this site.
Steam Hardware Survey for June shows 41% using dual core and 51% using 4 core processors. 92% of CPUs use 4 cores or less. Game developers do not develop games that only 8% of the population can play. The smaller the potential market, the smaller the revenue. They want to develop games that a lot of people can play. This is why many of the massive online games have very conservative PC specification requirements. So by the time that 8 or 6 cores becomes the majority of CPUs, the 1700 will be a dinosaur in CPU years and will require an upgrade to play AAA games.
With that being said, if you do anything other than gaming, then the Ryzen chips are definitely the way to go. They are very good chips and have an incredible price vs performance ratio. But if you want to quintessential gaming chip (all be it expensive) available right now, then the i7 7700k is the chip to buy.
If I were building a gaming PC today and money mattered at all, I would go with the Ryzen 1600 and use the money I save to add on to a better monitor to pair with my 1080ti. Using a 1080p monitor with a 1080ti is like driving a race car in a school zone all day.