The Ryzen 7 2700x turbos to 4.3ghz
The Ryzen 7 2700 turbos to 4.1ghz
This isn't exactly the whole story though. Their single-core boost clocks are not too far apart, but there's a significantly larger difference between their multi-core boost clocks. See this chart near the bottom of the first page of Tom's Hardware's 2600 review for a better idea of how the various Ryzen processors boost clocks compare...
https://www.tomshardware.com/reviews/amd-ryzen-5-2600,5625.html
You'll notice that both boost to reasonably similar clocks on a single-threaded load, within about 5% of one another, but they diverge as soon as you start loading multiple cores. The 2700X remains at a similar performance level, around the 4.1 to 4.2 GHz range, but the 2700 drops into the 3.5 to 3.6 GHz range, giving the 2700X over 15% higher stock boost clocks with multi-core loads.
I agree that if one is going to overclock, the regular 2700 is arguably the better value though. If there's any binning going on, the differences seem to be quite minor, and both processors should overclock to similar levels. You can get a 2700 with a better aftermarket cooler for less than what a 2700X costs, and get a similar level of performance with cooler and quieter operation. The 2700X technically can overclock as well, but doesn't provide much headroom for overclocking since the stock clocks already get most of the available performance out of the processor.