Sorry, I really don't have a personal opinion regarding the best SON in P'burgh...specifically if you prefer a diploma program.
If you are looking for a school that will give you the best shot at becoming an RN, I think you should look at the school that has the best balance of didactics, clinicals, and higher pass rates. BTW, as a diploma grad who immediately went on to finish a BS in nursing, the theory that diploma programs are superior is just not a true generalization. They aren't necessarily inferior, but they don't win any awards, either.
You really shouldn't sell short the UPMC programs...besides, they now run four of the nursing programs
(St. Mag, Shadyside, Univ. of Pitt, and now Mercy). Additionally, they often offer great tuition payback programs if you attend one of their schools and then work in one of their facilities for 2 years.
You will get just as good an education at WP...but I would be suprised if they'd let a new grad go directly to their burn unit.
In the end, we all take the same exam, so I suggest pursuing the program that best fits your personal needs and schedule, and don't limit yourself because of personal dislike for a name (i.e. UPMC). You may find that a ADN program like CCAC, Butler, or Westmoreland might fit your needs best, but no matter what you do, I highly encourage you to return and finish the BSN.