OK, Take away the view of the city skyline - and how much better is it?
I'm sure a big part of it for me personally is that I grew up in Pittsburgh and PNC has many architectural elements of Forbes Field, but I do think the sightlines and viewing angles are better and the concourses are bigger (might just be fewer fans!).
Overall PNC has more character or a nicer feel about it, e.g. the "north side notch" and a more open design. I don't know why, but Nats park feels plastic or artificial to me, kinda like a lego set or something. Of course, I'm not advocating sillyness like the hill in Minute Maid, trains, or anything like that!
All in all, they both beat the heck out of RFK, Three Rivers, Veterans, etc!