Responding a bit late as I just saw this now. I've done a fair amount of riding in the Eugene area and it is great. Just a few minutes to get to hilly, low traffic roads outside of town with a big variety of roads (hilly, flat, paved, gravel) available. There is a lot of riding past vineyards and farms, and through fir forests in the coast range. It is nice most of the summer, and rains quite a bit October through May. Usually the rain is more drizzly than downpour, and temps in the upper 40s so as long as you are OK with that, it rarely forces you to train indoors.
The town of Eugene is heavily influenced by the University of Oregon and has a reputation for being very liberal. This is true, but it is also mixed with a rural, logging town feel in some area. I like it a lot.
The racing scene is Eugene and Oregon in general is good, but has been shrinking quite a lot over the last few years (like elsewhere, cross and fondo/gravel fields are still quite large). You can see the race schedule and looking at the results pages you can get a sense for field sizes at obra.org. Most races have maybe 10% more starters than listed in the results so it will give you a good idea. A lot of Oregon riders also race in Washington which has a little bit bigger and faster fields, but most races will be 3+ hours from Eugene.
I know nothing of Irvine.
Chris