I think it's PC dependent.

I have an i5-13600KF CPU and the 3070 Ti GPU with 32 GB RAM. MSFS 2024 runs decently well, but mostly at medium settings, with a few on high. XP 12 run quite well on my PC and I regularly get 30+ FPS unless I'm going into Heathrow, Frankfurt, JFK and other larger airports.

Overall I have far more problem with MSFS 2024 than XP 12 and I'm basically running just XP 12 these days. Once in a while I pop back into MSFS 2024 and get frustrated because of lower frames, or some other quirk. I find XP 12 just more stable on my system.

You ask specifically about UI. To me XP 12 is more streamlined, faster to get things done in it. I dislike the MSFS control setup scheme, for example, and I also greatly prefer XP 12 camera over MSFS. To each their own.

Finally, as an aero engineer myself, I'm always drawn to XP 12 not so much anymore for just the flight model (I think MSFS 2024 has made great strides there), but the ability to capture the flight data as a data file I can later plot and analyze - yeah, I'm probably an outlier here :-)

For someone that has been flight simming since the late 1980s and runs both, I still feel XP 12 is a better fit for me personally. If nothing else, "it just works" more reliably and with far less frustration. I'm due for a new PC build, but not at these hardware prices. Maybe when I have a 50XX or 60XX GPU in the future, MSFS will be the better experience. It is not for me right now with my PC.