I think Detroit is a pretty good city for bicycling, but it isn't so good for people who can't drive at all. People who need to commute are the most disadvantaged as relatively little of the employment is transit-friendly. If you can't drive and need to work, you are probably sensible to think about moving.

Portland is a nice place and is pretty much the opposite of metro Detroit in terms of urban planning, but partly because it has a reputation of being nice it attracts people who have no concrete reason for being there, and consequently generally has high unemployment, though not so high as Detroit.. If I wanted to be car-free and employed, I might look a different city. If I especially liked Portland, I would go there anyway and take my chances. Good luck with whatever you decide.