Full disclosure - I purchased the 7800X3D in the end but I’m still intrigued by the performance of the 7900X3D.
Benchmarks which include both the 7800X3D and 7900X3D are hard to find so admitedly this is a tiny data set but in one of the few apples to apples comparisons (Tom’s hardware), the 7900X3D is slightly faster for MSFS than the 7800X3D.
While the argument could be made that the 7800X3D is faster in just about any other game, a counter argument could also be made that the difference in those other games is unlikely to be noticeable in real-world use and the 7900X3D beats the brakes off of the 7800X3D for any workload besides gaming.
I was lucky enough to get the 7800X3D at list price but I see now the street price at some resellers is higher than the price of the 7900X3D. At equal pricing my money would go to the 7900X3D.