If I had to pick one company it would have to be Bally for the art or Williams because they are easy to find parts for and hold up pretty well and are typically the cheapest. I know a lot of guys like Gottlieb but to me they are the most cheaply built of the "big three". Chicago Coin made some great arcade machines back in the day but for some reason they just never seemed to "get it right" when it comes to pinball machines.

I really don't think you can go wrong with any of the "big three" but I think if I was in your shoes I would pick two Bally & two Williams and not worry about having two different brand machines. You don't need to stock all that many parts for EM's anyway so why not mix it up some?