Coffee Lake 8700k vs Ryzen cpu's

I see both as being more than enough for RPCS3 once we get more optimizations, due this I would pick whatever fits your budget and needs more outside of RPCS3. Personally I would recommend Ryzen, but if you can't buy now then I would wait for gen2 it's going to be 7nm while cofee lake is 14nm (same as gen1 ryzen) so it will be quite attractive price/perf.