I know you'll likely be talking to Don tomorrow, but I'm sitting here looking at your figures and wondering if you missed more than those two walls you mentioned later in the thread. R13 and R19 in Philly? Are you talking floors, walls, or ceilings? I would think you'd want more than R19 between you and an attic in Pennsylvania.
Check to be sure you accounted for every window, every door, insulation between first floor and basement, ductwork in conditioned spaces or not, etc.
Roughly speaking, 5 tons seems too large for your square footage and geographic location, but 3 tons would on the other hand seem to be pushing undersized territory.
I'm sure Don will walk you through it fine. He's got a great product, and you can't beat that kind of customer service.