I lived in nearly all points of London: North (Hendon), South (Peckham), East (West Ham) but never West. I also worked in Central London in a shop on Tottenham Court Rd.
London was the most fabulous and at times frustrating city I ever lived in. I love it to pieces.
So you have been to Pennsylvania. I am assuming you mean Philadelphia. It takes a bit getting used to. It's a very poor city but rich in history. Pittsburgh, where I am from is a much nicer city in Pennsylvania but its a bit far from the coast (like 8 hrs west by car). It has its own flava and quite a few Brit ex-pats love it.
Seattle is a very futuristic city with a HUGE music scene. It is always forward looking, progressive and open to the world in terms of influences. It's a unique city in America with its own unique culture. Hard to explain really. You'll just have to come here and check it out. It's well worth it.
Any place that produced Jimi Hendrix, Heart, Nirvana, and many other great artists and bands must be doing something right.
I moved here almost straight after London and it was the perfect place for me.