http://cpu.userbenchmark.com/Compare/AMD-Ryzen-7-2700X-vs-AMD-Ryzen-5-2600X/3958vs3956
If you have the money, go for the 2700x. If this is a pure gaming machine, I would go for the 2600x and save some money. Basiclly you will get slightly better single core on the 2700x, as well as getting 2 extra cores, but if you are doing gaming, a 2600x will do just fine i think, it just comes down to if you need the two extra cores