I'm currently planning a trip and have looked at both. CouchSurfing seems to rely a lot more on a sense of community, whereas Airbnd is more "business-y", which only makes sense given that CS is free whereas Airbnd is not. CS users are encouraged to maintain an up to date profile; the intricacy of some people's profiles rivals MySpace profiles of yore. Airbnd has a much nicer website.
Also, included in the CS-experience is the socializing – nobody wants to host someone who's just taking the "free couch"-part, so actually there is compensation included, but it is not money.
You have to pay for Airbnb but do get a bed? I do like the idea of Airbnb and hope to be able to try it soon.