Yep, the University Z might not allow you to have an outside job.
As others have said, if you want the option to go into academia, you've got to go with either X or Y. Presitge, fame and impact factor are the things that the acadame runs on. You won't be able to make up that difference from a less respected program by developing an outside network (part of what going into a well-respected, highly-ranked program means is that you have an advantage in building a good network, through your advisor's contacts and through the other graduate students that you work with). You'll be more likely to do better work and develop a stronger network at a place where all the faculty is research active, the students are interested in research, and the university makes it a priotity to support research. Also, did your potential advisor at University Z come in with tenure? It might be likely that that person (like the rest of the productive faculty) might leave University Z, possibly sometime in your tenure as a student. Were that to happen, what would that mean for you?
You, or your family, probably have more information about how much prestige matters in industry. I've got no information for you on that.
I would think, that it depends on your long term goals. IF you would use the degree to do research for the benefit of your family firm and maybe someday head it, University Z would be the best option. BUT if you want to have an option to quit your family business and go ahead on your own, University X & Y makes sense. But for that you would maybe lose 2 years of money.
That said, it depends on how badly you want to have that OPTION to quit family business. If you like what you do with your business and you really have grand plans ahead, University Z is a no brainer.
