The AMD Ryzen 7 5800X3D wins in 9 out of 14 comparable specs, with 1 tie. Its biggest advantage is L3 Cache (1500% better).
Core i5-6600K wins in 4 specs (MSRP: 46% better).
Recommendation
Choose Ryzen 7 5800X3D if you want the best overall performance — especially l3 cache.
Choose Core i5-6600K if msrp matters more to you.
