I just compared the ug extensions for the ks 60 and ks120 side by side on ekat and I spotted a few differences:
The scale for the stops (which doesn't matter if you only use it with the mft3), the type badge (also not important)
But also the bottom claw of the clamp that attaches the wing to the saw, or in case of the ks60, to the plastic adapterplate. (which could be a problem)
I don't know how much the price difference is at the store, where you're looking to buy, but the recommended retail price from festool is the same for both type of wings.
You can order the claw for the KA-UG for the ks120 for € 5,24 (Dutch price), if you decide to get the wings for the ks60, and they wouldn't fit.