I'm torn between getting either of those CPUs. 5800x would be at 389€, the latter coming in at 549€. I'm only using my build (rtx 3070, x470, 3600) for gaming in 1440p, so 12 cores are hard to justify. Then again, the 5900x has better price per core, cooler, better binning, maybe more future proof. On the other hand it's still at MSRP while the 5800x is already discounted.
Search just about any site that reviews CPU/GPU performance, and you will see the CPU has less to do with game performance at and after 1440p.
If you are purely gaming, some say you could get away with the 5600x chip. Multi-core game support still has to go a long way, with most games benefitting from 2-3 cores at most.
If you are interested in the future proofing, my preference would be the 5900x with a good liquid cooling solution.
Gamers Nexus did some good coverage on the 5000 series chips.
On availability, it seems there are more 5600/5800 series chips available, although the 5900s are starting to pop up more frequently.
Since AMD agreed to allow access to the Smart Access Memory feature, your 3600 would work and possibly bump up a few frames, but you would need to upgrade to a 500 series chipset mainboard.
I guess it depends how much you want the latest and greatest. Probably worth waiting to see what the next gen offers, as well as when the market availability improves in the post COVID/chip shortage era.
Hope this helps!
After much thought and checking reviews out,I went with the 5800X and a Crosshair VIII Dark X570,Didn't see the need to spend $200 more for the 5900X.