I'm choosing between a used Ryzen 9 9950X for €339, a new 9950X3D for €616, and a sealed 9950X3D2 from a private seller for €700. The build has an RTX 5080 and is mainly for Unreal Engine development, compiling, gaming, and some neural network work.
Is the extra cache worth the premium for frame times and CPU-heavy games, or would the 9950X be the better buy with an upgrade later if needed?
At those prices, I'd take the 9950X. It's a strong fit for code and shader compilation, and the savings over either X3D option are substantial. Extra cache can help games, but that doesn't mean it will help every development workload by the same amount.
Keep the upgrade idea as an option rather than the reason to buy. A later AM5 CPU might be appealing, but its performance, price, and compatibility with your board aren't things you can count on yet.
The 9950X3D is the one I'd consider if gaming smoothness matters enough to spend extra. Look for benchmarks of the actual games and development tools you use, including 1% lows, not just average frame rates. Cache benefits vary a lot by workload, and a GPU-limited game may show little difference.
The retailer warranty is also worth something compared with a private sale, although whether it's worth €277 on top of the used 9950X is a personal call.
If you're already willing to pay €616 for the 9950X3D, the extra €84 for the X3D2 is a smaller step. I'd still want relevant benchmarks before paying it, though. The highest-end chip isn't automatically the most useful one for compiling or neural network work.
Also compare the private seller's proof of purchase and warranty terms with the retailer's. Sealed packaging alone doesn't answer that question.
Durnel.84:
And compared with the 9950X, it's still €361 extra. That makes more sense if the specific gaming gains matter to you than as a guarantee of future-proofing.
For the thesis work, check the memory requirements of the models and tools you plan to use. An RTX 5080 has 16 GB of GPU memory, which is a separate consideration from the CPU cache. The cheapest CPU won't fix a model that doesn't fit, and the priciest one won't guarantee it will.
Same goes for the rest of the development build. You've already budgeted for 64 GB of system RAM and a fast 4 TB drive, so check your actual project needs before assuming CPU cache is the part most likely to hold you back.
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nearly.in_the_margin:
The used price looks good, but make use of the seller's testing offer. Check that it runs reliably under load and ask what purchase or warranty documentation is available.