OGS might be more accessible to new players with one click sign in and a better web app, but I think KGS has a higher population of true dan+ strength players, and has a stronger "culture" around community reviews and studying.
It used to be even better, but there are less total people playing on KGS than the previous peak.
I've read about KGS but I've never figured out how to engage with it. (I know it to be the OG, srs bzns venue, though).
Just download CGoban, here is the website:
Most of us started who are still active on the server have been playing there 20+ years at this point. E.g. I myself (and many others) started playing after reading this article:
Out of curiosity, why do you like OGS more? I find the UX of KGS to be way more intuitive.