ChipFlip
Here's a used-market situation that trips people up: the newer Ryzen 7 9700X costs $245, while the older Ryzen 9 7900X costs $209.95. Fewer cores, newer architecture, higher price. The instinct is to take the twelve-core chip and save $35. That instinct is right about half the time.
What Zen 5 buys you over Zen 4
Better instructions per clock, improved branch prediction, and notably better power efficiency. The 9700X does more work per core and per watt than the 7900X does. In lightly threaded workloads and most games, it holds its own against a chip with fifty percent more cores.
It also runs much cooler. The 7900X is a hot chip that wants serious cooling to hit its boost clocks consistently. The 9700X is comfortable on far less.
Twelve cores still wins the jobs that need twelve cores
Render a scene, compile a large project, encode video, run virtual machines. Anything that saturates every available thread hands the 7900X a clear win that no amount of per-core improvement closes. Four extra cores is four extra cores.
If your workload has a progress bar that takes minutes, the 7900X finishes first.
Same socket, so the upgrade math is identical
Both are AM5, both use DDR5, both drop into the same boards. Neither locks you out of anything the other allows. That removes the platform argument entirely and leaves this as a pure question of what you do with the machine.
Bottom line
Gaming machine, or anything where responsiveness matters more than throughput? Pay the $35 for the 9700X and enjoy the lower temperatures. Doing real multithreaded production work? The 7900X is $35 cheaper and meaningfully faster at it, and the extra heat is a cooling problem you can solve.