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.
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:
For KGS you had to bring your own client, like cgoban, if memory serves. They have a web client now, but the Java client was what I played on for while.
I find everything about OGS superior to KGS except for the quantity of strong players. In particular, its active development, community involvement, and modern tooling all make it more appealing. KGS is very closed source, running ancient software on ancient hardware, very static, very DIY and individual. It's about the game of go and little else. That is admirable in some regards, but not suitable for a growing niche community that is Western go.
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.