Should i buy a) Ryzen 5 3600 with Msi 2080 Ti Ventus 11gb or b) i7 9700K with EVGA 2080 XC Ultra.
I believe a) will give better gaming fps but unsure as never had and amd cpu before.
Can you post the rest of your build? What's your overall budget? What games are you hoping to play on this, and will you be using the PC for anything else?
Broadly speaking, I would expect the first pair to give you better performance, given that the GPU is more often than not the more important factor for gaming. That said, unless you are getting really good prices on these components, I would not buy either of these combos.
The RTX 2080 Ti (which I'm still seeing as costing over $1,000) is expected to be eclipsed by the RTX 3070 which has a $499 MSRP. Granted, availability will probably be scarce, but I still would rather wait a couple months than throw hundreds of extra dollars at the outgoing generation. AMD's Big Navi GPUs are also being announced at the end of the month so it's worth seeing what they have to offer.
It's the same story with the CPU. Again, unless you're getting really good deals, you will probably be better off buying newer hardware. The Ryzen 5 5600X is set to drop at the start of next month and promises a considerable performance boost over the outgoing model. The i7-9700K meanwhile is already a generation behind, and while still a good CPU, is equaled now by the i5-10600K.