3.0 Ghz against 3.5 Ghz means roughly -14% performance.
But you have to keep in mind that if only one or two cores are busy, then Ryzen goes into boost-mode, and works on increased frequency.
Look at some reviews. They mension that boost frequency of Ryzen 1700 vs Ryzen 1800X is 3.7 Ghz vs 4.0 Ghz, which isn't much.
Also, there are plenty of single-threaded benchmarks (including games) on the net, where you can compare different Ryzen CPUs.
Of course, not every program can make more than six threads working simultaneously, so your +33% boost from two additional cores won't have effect most of the time.
I have bought Ryzen 1600 recently.
I was considering Ryzen 1500X too, but it's increased single-core performance seems too low for me.
Moreover, I have read many times that Ryzens without 'X' can often be overclocked to almost the same frequency, although no guarantee here and it depends on luck.
In fact, I often compile TDM using my home computer, and having 6 cores against 4 cores for me means almost +50% compilation speed
P.S. And when looking at benchmarks, remember that 80 FPS vs 75 FPS is not a major difference