VKB-SIM

I'm opening my wallet for another thrashing courtesy of you fine people, but I'm not certain which UCM mount suits my needs.

Should I use the UCM S, or UCM L if I plan to use a side mount with a 100mm extension?

Side note, I saw in another topic that a 50mm extension is in the works, should I hold off and go for that? I hear the 100mm is a tad heavy for space sims.

I will be using a UCM s for my left gf/kosmo with no extension (used as a throttle with no spring), a side mounted UCM s/l for my right gf/kosmo, and a UCM k/m beside the right stick.

Depends on you height and the height from your chair to the floor. If your position is higher than average then you could be able to use a UCM-S with a 100mm extension. If you're lower (shorter or low chair) than it'd be definitely an UCM-L.
Problem with using a long extension in space sims is not that they're heavy, but the distance your hand travels along the axis range is longer than if using no extension. Using an extension gets you more precision in aiming, but you'l become slower in strafing when dogfighting in small ships.
I, for example, prefer the Kosmosima and F-14 grips in space sims, as the hand rests low, closer to the base than when using the MCG grip.