My two cents on the subject:
For me it is very clear that the 50mm extension makes the most sense for the Warthog stick.
Previously I had mostly used it without an extension, flying DCS aircraft including F-16C, F-14, F-18, Saab AJS-37 etc. However, I always found it a bit too stiff for precise flying without getting tired. I use a Monstertech table mount. I then tried a 100mm extension for the stick, but it only made sense for helicopters (Black Shark, Huey, Gazelle). Unfortunately, with the 100mm extension, the self-centering of the stick was a bit too slow, which was very annoying, and useless for fighter jets. Constantly removing or adding the extension or changing the spring wasn't ideal either when you often switch between helicopters and fixed-wing aircraft.
Since I started using the 50mm extension, it stays on the stick permanently: fixed-wing aircraft can be flown very precisely without my arm or wrist getting tired, and even for helicopters the stick is long enough for good control & immersion. I don't even have to adjust the height on table mount, I just move it from my side to between my legs. The constant changing of extensions has now come to an end! I can instantly switch from helicopters to fixed-wing and back. I also don't have to change the spring anymore, the stock spring is perfect for the 50mm longer stick. I'm super happy with this solution. I use a “Reyann Aluminum Alloy Extension Rod 50mm” from Amazon which I can recommend.