For terrain, as mentioned, Springfield is mostly flat. Kind of an odd "mill town". It has some good roadside bike paths, as well as a few semi-through streets with low traffic that are designated for cycling.
Eugene is a little more affluent, but still mixed income, with a mix of hills and flat. But, most of my riding in Eugene is also flat. Just find something that meets your needs.
Eugene has some of the best bicycle bridges I've seen so far, with several bicycle only river crossings mainly connecting the two riverside bike paths.
There are a few "bridge people", but generally fairly scattered around. Personally, I've never had any negative encounters with them.