For me, Williams are the easiest to work on. They have really handy visual guides in the manual for switches in all of the relay stacks, and the score reel design is simple.

Gottliebs changed their architecture when they dropped the reset bank and went to the Ax/Bx relay setup in 1975, so it's possible to have all Gottliebs, but with a different repair processes between the two styles. You could do all post-1975 Gottliebs or all Pre-1975 Gottlieb games to avoid this though.

If you're looking for parts supply, I think Gottlieb has the edge over William's. Pinball Resource ALWAYS has what you need for anything Gottlieb EM.

I disagree. CCM games are pretty terrible. The cam that holds the score motor disk is weak and often breaks, and the relay stacks are all made from cardboard that warps over time, and the score reels use a wiper disk plate for the 0-9 position switches. Couple that with oddball parts for drop targets, flippers, connectors (they use a molex style, not a jones plug style), and lackluster gameplay, I have a really hard time recommending Chicago Coin games to people. They just don't seem well built to me.