Eugene is probably the crunchiest of the lot, being a college town + having a long standing tradition of being alternative in many ways. I don't think there's cohousing there. It has all the advantages and disadvantages of a college town; it's big enough to have OK public transportation, but small enough that it's not really a city.
Portland is obviously a city -- there are half a dozen cohousing projects around. There's much more of a mix of people -- everything from fairly conservative to extremely radical. Homeschooling is common, unschooling is definitely known, but I don't know much about the groups as my kids don't fit that bill. Where you choose to live will have somewhat of an impact on your community. Each area has definite characteristics, and there are a few neighborhoods that I simply wouldn't live in. I like the Portland area because it's more diverse and has lots to do. I'm a city girl myself and so feel comfortable in the city and I like the range of transportation options.
Bend is on the other side of the mountains in the high desert. It's much sunnier (even in the winter), but it's also a bit touristy in the town itself, IMO. (I just got back from a weekend in Bend.) It's got much more of a rural feel to me, and yet sometimes the traffic can drive you bonkers during tourist season. I don't know about cohousing, and I suspect you'd have a decent, if smaller, unschooling community. It's a cool place to live (ranches and mountains close), but it's not great for getting anywhere else.
Anywhere you live in Oregon, I'd want to make sure that your income/employment opportunities are solid. Unemployment is always higher here than elsewhere in the nation. It's got much more of a boom and bust economy. It's never, as far as I can tell, figure out how to pay for basic services such as education and roads. On the other hand, it is pretty libertarian, and so live and let live. But I wouldn't come without a job lined up, and some indication that your skills are in demand.