I have both and haven't noticed a difference in performance, but I haven't actually done a proper comparison test either.
They do feel different, though. I find the flatter tops of the Universal bodies, combined with the full covers, a bit more comfortable than the MAFAC levers.
Here's the MAFAC-Universal combo on my Toortelboom:
I like MAFAC cantilevers and run them on several of my touring bikes and, because with those period-correctness is less of a consideration, I like to combine them with early Campagnolo Ergo levers. Besides their ability to shift vintage 5 and 6-speed transmissions, they are more comfortable, make braking 'from the hoods' easier, and there are no brake cables to get in the way when accessing a front bag.
I'm currently running Brifters on my Roberts, but I wanted to experiment with non-aero and friction bar end levers on the Bridgestone that's going to replace it. I've used aero and friction bar ends before and I've bikes with non aero levers (both Mafac combos) so it might be interesting to try and different set of levers. I suppose replacement hoods and maybe other parts are more easily available for Mafac, but I know Tre Emme do the replacement Universal hoods.
Thanks. Somewhat put off by the prices of décaleurs and my wanting to be able to switch bags between (many) bikes, I decided to cobble something together using an 8 euro Minoura SwingGrip and 20 cents worth of aluminium tubing:
The bag is held by its own straps and as the support is adjustable it fits a wide range of bikes and racks. Not quite the René Herse or Jo Routens level of finesse, but it works well enough.
Two leftover bits of cotton bar tape keep the bag from sliding. A precaution rather than a necessity, though.