Or go for 9th gen some people say they are almost identical in performance is that true? While on papers/internet I9 10th is 11% stronger

For some things the 10900K is quicker, for others it's about identical. Hell, for some things an old quad core is identical, if you're talking about, for example, those games which are entirely GPU limited at high resolutions. So the right CPU for you depends totally on what you will be using it for.

I would say that unless you know you need a 10-core i9 then you probably don't need one. If you are only gaming and aren't doing heavy duty creative work / professional streaming etc. then an i5-10600K or at most a i9 9900K or i7-10700K would likely be a better buy for you.

If you don't already have a 9th gen motherboard then I wouldn't say it makes a lot of sense getting an i9-9900K in any case.