I've used a 12600k and a 12900k. In normal desktop usage and games, I wouldn't say there was a noticeable difference. There are plenty of reviews out there to show you exactly how much difference you'd expect to see.
As an owner of a 12700K who mainly plays games I would go with that. I've read good things about the 12600K as well as a gaming cpu, so in reality you can't go wrong with either. I will say you'll see quite a performance boost from your 9600K.