Both CPUs feature a 65 W TDP and support 24/16 PCIe 4 lanes. The Ryzen 9 3900 comes with 12 cores and 24 threads clocked at 3.1 GHz (4.3 GHz boost) and integrates 70 MB total cache memory, while the Ryzen 5 3500X gets only 6 cores and 6 threads clocked at 3.6 GHz (4.1 GHz boost) plus 35 MB total cache memory.
https://www.notebookcheck.net/AMD-launches-Ryzen-9-3900-and-Ryzen-5-3500X-CPUs.437610.0.html