A 9900 and 2080 doesn't need 850w either. (Though it's not a bad idea to get that if you are OCing.) Your system choices seem to be a bit scattered. Have you bought all this already?
Seeing as the 2060 doesn't SLI, I wouldn't have bought it as a placeholder card. If you have another pc to move it to that's cool. Under gaming load your system is going to pull around 500W stock. A good 650 or a 750 will work fine. 850 is really only needed for heavy OCing. I mention all this because you seem to be worried about cost. Or else you have have bought all this already. Yes, a seasonic platinum is better than a CM gold. This shouldn't surprise anyone. You still haven't said prices so I'm not sure it's really worth it. I can't imagine spending 250 anything on a psu but I'm in the USA so my prices aren't the same as yours. I suspect the cheaper one will be fine. But I'd also get a better 650 or 750W.
yes i have old PC i7 cpu with old GC 580gtx
and i plan to add the 2060 in that and get me 2080 or 2 x 2080
i am not worried about cost.... i just don't have cash right now to get me 2080
but prices are a loot cheaper in USA compared to EU or any country around EU
here is build and prices:
CPU: i9 9900k - 665 euro
m. boar: Gigabyte Z390 DESIGNARE - 365 euro
cooler: kraken x72 - 220 euro
Graphic C: Gigabyte nVidia GeForce RTX2060 6GB, N2060GAMING OC-6GD = 485 euro
HD: samsung evo 970 nvme..500gb - 165 euro
ram m.: 2 x 8Gb ram 2666 - 160 euro
PSU: Corsair HX850i , CP-9020073-EU, PSU 850W - 255 euro
Case: thermaltake A500 - still waiting distributor for availability and price
also in this build i will add 2 x SATA HD for storage