short term like 2 years probably 8cm femurs is better but you have to account for the fact that doing femurs will inevitably misalign the axis of the femurs and effectively make you walk differently biomechanically and may result in arthritis. ideally 6cm femurs and 4 tibias for long term in terms of like recovery in 5-10 years+ is better for long term recovery and lifestyle. do not misunderstand doing femurs at all will mess up the axis even 1cm but less reduces that misalignment and 6 + 4 is huge amount of lenghtening no matter how one tries to twist it and you will not be the same ever. athleticism idk i feel like in the long run doing 6 + 4 is still better athletically but its just a guess.
my understanding is that in the long long term, 10 years + it is better to do quadrilaterals just because of biomechanics. short term definitely single segment.
Arthritis in LL can be promoted ("due" is a too strong word) by: -joint pressure, which however decreases gradually anyway; -after a tibial lenghtening (probably beacuse an increased mechanical lever, but this is my thought based on statistics); -due to deformity of the knee. Lengthening over the anatomical axis with a nail leads to a valgus deviation BUT:
-The deviation got is very light
-You have to consider that everyone has a different "starting point"; someone has a slightly valgus knee at the beginning, others can have a slightly varus knee at the beginning.
Anyway, femur lenghtening has be proven to be better for athleticism recover, but tibia is a good option if your tibia is pretty short at the beginning or if you don't have much money, because external tibia is a safe procedure unlike external femur.