You really cannot make a fair comparision between two companies with NIB machines as where one company is still making them and the other is long gone. I'm sure as some state todays new Sterns are better, well maybe so however lets be fair were the Sterns of the same time frame just as good as the Williams/Bally units? I'm sure they had their issues as well. So to say Sterns is way better because of issues W/B had over a decade ago is an unfair comparision when they have 12 more years of advancements on their side and what would Williams/Bally quailty if it were still here today? No one can know for sure.
I'd still take a 20 year old loose W/B machine over any new Stern unit. There is a quality feel difference, to me anyway.
Let's go a step further: Would Sterns even exist today in the pinball industry if Williams/Bally were still around making machines? There's a thought.