The AMD Ryzen 9 9900X3D wins in 9 out of 14 comparable specs, with 2 ties. Its biggest advantage is L3 Cache (700% better).
Ryzen 5 8500G wins in 3 specs (MSRP: 70% better).
Recommendation
Choose Ryzen 9 9900X3D if you want the best overall performance — especially l3 cache.
Choose Ryzen 5 8500G if msrp matters more to you.
