I though I wouls weigh in, as I am almost exclusive a mmo player like wow or ff14. if you look the very few articles focusing on MMO in particular, you will notice they scale well with 3d cache, with MMos more player on screen means more fps loss, generally a scenario you will see often in a MMO. Though the "faster single core" rule holds true, that has its limits in the ff14 charts from gamers nexus for example 7800x3d is just behind 13900k despite slower clocks. ( weird that an amd console game heavily favors Intel)
I recently went from 5900x to 5800x3d, I can see 1% lows are much better in wow as an example in mass world events I have gone from 30fps ( winters grasp woltk) to 60fps plus with noticeable less micro stutter. I run at 1440p 165 hertz on 6800xt for reference.
So in my opinion unless you are going 13900k or 14900k to the brute force of fps with clocks the 3d chips are way forward for mmos in particular.
Now to original question I would go for 7800x3d it is more future proof (as much as one can be) but also higher clocks speeds + cache will help in mmos but also for games that don't care about the cache. The 5800x3d draw back is that even with pbo tuning on boards that allow it (some msi and asus) . You will likely never see an all core clock past 4.45ghz.
I hope that helps as I was in a similar situation, though 5900x is great chip, the 5800x3d was best for my mmo use case. In next few years I will sell my current parts go for 7950x3d cause why not.
Below are some sources of data, mmo players have run on there own for x3d tests. There are surprisingly few
The AMD Ryzen 5800X3D is finally out! A game changer! literally... I have tested it and compared to the most commons rivals for World of Warcraft. This is a ...
youtu.be
There is a link to the chart at the top of the reddit. At it best on ff14 5800x3d is 40% increase at its worst a few percent minus.
