It seems like http://www.couchsurfing.com/ is a bit more mature than airbnb.com, but airbnb.com looks more technically competent.
When I read "mature" I thought you meant in demeanor, but now that I look at the site, I am guessing all you meant by it was ... older/more established? I like the inviting-ness of airbnb, and sheer simplicity. For me it is much more welcoming, and thus motivating (to go out and explore) than the vibe I get from couchsurfing. to each his own.
Yeah, I mean that CouchSurfing has an existing community that has already gone through a lot of the growing pains that social-network sites go through, and come out with a reasonably functional group; it's pretty rare for people to get robbed or raped through CS.