Have lived in London and (originally from) Paris. My US experience is limited to a few cities, so I can speak about characteristics that yr fiancee (I assume it's "she") may enjoy:
small cafes, restaurants or brasseries, creative shops (any thing from cheese to Xmas ornaments), many venues (small & large) for art exhibs and music, geographical & architectural beauty for nice walks... Standard shopping is so easy in the US I don't think you should relocate for it.
If she's European, however, a nice house and peace & quiet will do! There's also lots of beautiful landscape readily available in the US -- no worry there. Proximity to the sea is also a very strong argument!
NY and Boston are large cities most Europeans feel at home at, particularly NY: in some respects it looks like '30's Europe (rather, what '30's europe would have liked to look like had we had the will & the means!)
Cheers