7800X3D or 7950X3D for gaming and work at the same price?

The 7950X3D is not automatically worse for gaming. Its cores can be managed so games use the cache-equipped chiplet while background tasks run elsewhere. Older setups had more scheduling problems, but updated motherboard firmware and AMD chipset drivers have improved the experience considerably.

If your work involves rendering, compiling, heavy editing, CAD, virtual machines, or other heavily multithreaded tasks, the 7950X3D is the stronger choice because it has 16 cores instead of 8. In most games, both are very close, so getting the 7950X3D at the same price makes sense if you can properly cool it.

For a gaming-only build, I would lean toward the 7800X3D. It is simpler, generally more efficient, and avoids the small scheduling and latency complications of the 7950X3D's two CPU chiplets. The gaming difference is usually minor, though, especially with current BIOS, chipset drivers, and game scheduling working correctly.

At the same used price, I would choose the 7950X3D unless your main priority is maximum simplicity and low power consumption. It performs similarly in games while offering substantially more performance for demanding work. Just make sure the seller is trustworthy and budget for a capable cooler, since it can draw more power and produce more heat.