Either choice would not be wrong.
My suspicion is that Coffee lake will not be that much of an improvement over Kabylake (as it is still built on 14nm), insofar as Skylake was not over Haswell, and Kaby lake was simply a clock increase. Plus delidding and overclocking the 7700k up to near the 5ghz range should keep it relevant with whatever Coffee Lake offers.
If you can afford to wait until the middle/end of Summer (August is supposed to be the annoucement) then waiting could get you a Coffee Lake i5 6/6.
The caveat is two fold.
1st, we don't know if the clock speeds of the i5 6/6 will be the same as the 4 core counter parts. The X-series chips with more than 4 cores have always been slower than their consumer 4 core counterparts.
2nd, we don't know what the price will be. It could turn out the 6/6 is nearly as expensive (or more than) the 4/8 i7.
To me that is a lot of "ifs" to put off upgrading now.
You would not be misserved by an i7 4/8, and in gaming applications (no streaming video editing, etc), I would theorize that you would not notice a substantial difference between a 6/6 and a 4/8 or 6/12, for at least as long as your build stays relevant, probably until at least the generation after Coffee lake (2+ years away).
The optimization of Ryzen is unlikely to overtake the inherent limitations of the current line up, namely, the lower IPC and Clock speeds compared to Intel.
Zen2 may correct both of those issues (or not), and it is supposed to use the same motherboards as Zen1. So in theory if it does, you could simply upgrade the CPU down the line. BUT you will likely not be well served by the additional cores and threads (beyond 4/8) which you will not use if you are only gaming. It is true that more games take advantage of >4 threads, but I think it is unlikely it will improve to such a degree that you will be limited by 4/8 by the time you are ready to upgrade again.
The only benefit Ryzen has at this point is that it is and will likely be a cheaper upgrade path.
But this only matters if you plan to upgrade the rest of your system. An i7 with a 1070 is not going to perform any noticeably better than a 1600 with a 1070, but it will cost more. You are probably looking at a 1080 performance or above to fully take advantage of the 7700k.