Here are the determining factors of where to live.
1) Traffic - Traffic is horrible going into the city during the morning commute and horrible going out of the city in during the evening commute.
2) Quality of life - This is what type of neighborhood you want to live in. Do you want to be able to walk places easily, be in a cleaner area, etc.
3) School system - Kids? Then you want to take into account the public school system. The Portland Public school system has some great schools and it also has some god awful schools, so you want to buy in an area with good ones obviously.
4) Urban or Suburbia - Which do you prefer?
5) Income tax - If you live or work in Oregon you have to pay Oregon Income Tax. So your girlfriend even though she works in a state that does not collect income tax, she will have to pay the state of Oregon Income Tax if you decide to live there. Sorry dude but working in Oregon you're hosed either way.
I grew up on 11 acres of land north of Vancouver, 4 wheelers, shot guns, BBQ, etc. Basically a red neck... I married a woman who grew up 2 subway stops from Fenway Park in Boston, MA. So since I'm a guy and everyone knows the woman gets what she wants, I live in Northwest Portland and am a "city guy" now.
In all honesty I love it in Northwest because I work in Southeast Vancouver and my drive is always counter commute. Takes no more than 20 minutes to drive the 17 miles. Getting to and from Beaverton/Hillsburitto is pretty easy if you take the Max Train, but if you are starting in the city where I live, then "in theory" your drive would be counter commute as well.
Northwest and Southwest Portland (in the downtown area) also have access to the best schools in the Portland Public School System. Again I don't know if this is a factor for you, but it does help the value of your home.
If you want to live in a more suburban environment, then I would probably go with Beaverton and make your girlfriend drive over the west hills either using Barnes road or Cornell. The drive for her North of downtown will be cake, it's getting into/through downtown that will kill her.
So pretty much you can't lose if you live in downtown, unless you hate the urban atmosphere.
Hope this helps!