Essentially the only difficulty change from the vanilla values is
ufoFiringRateCoefficients: [100, 85, 70, 55, 45]
The vanilla values are:
ufoFiringRateCoefficients: [100, 95, 90, 85, 80]
This affects the rate at which UFOs can shoot. This was nerfed a bit for Genius and Superhuman in a recent update.
There is very little difference between veteran and genius. Both spawn the same amount of aliens, the minor differences relate to accuracies, retaliation chances, etc. Either one should be good. The biggest consideration would be whether to play superhuman. If you've already played the mod that's what I'd suggest.