It's simply my opinion, but Williams EM games are in the main easier to work on than Gottliebs.
Some of Gottlieb's relays and score reels can be a real dog to work on and get adjusted correctly. Score motors, too, once Williams went to the reel type from the platter type.
Gottliebs generally, but certainly not always, were better at game play. But I've never found them to be easier to work on, nor easier to keep dialed in once you have them fixed.
Gottlieb parts however are much more plentiful than Williams.