Well, this probably comes too late for your trip to Pitt, but I thought it might help anybody else who reads it.
Polish Hill is a pretty rough area. I think you'd feel isolated b/c there aren't that many students there.
Highland Park is really too far for an everyday commute. It would wear on you, and you'd probably feel isolated.
I spent a lot of time researching, but it didn't help that much b/c my opinion changed completely when I got to Pittsburgh and really looked around. I rented a three-bedroom in Shadyside. I think that Shadyside and Sq.Hill are really the places to be. Regent Square is nice, no question. But it's perhaps not as fun as Sq.Hill or Shadyside. And it's a lot harder without a car.
If I were you, I'd think twice about renting a 1 bedroom or studio -- basically because it will cost you too much of your student budget. You could either share a two bedroom for 400/month or have your own place for 600/month. And if you took the latter option, that means you'd be spending about half of your entire student budget just on housing (i.e. what Pitt gives you for "living expenses," which many, if not most, students will be solely counting on).