For a globetrotting in Europe, which one do you prefer and why? Or you do prefer another service?
If you are strapped for cash but have ample time to prepare, couchsurfing is great. You can find a good host, chat them up, make a connection, and likely have a wonderful time. It can be a lot better than a hotel/hostel.
Of course, there's always a chance to run into someone creepy, which is no fun. But that's what vetting, talking, and preparation are all for.
Airbnb is awesome for finding something affordable quickly. Plus more often than not you get people / companies who are registered businesses and can help you with visa stuff by sending a voucher or booking confirmation (if that's relevant to you, to me it very much is).
I have often went on Airbnb to find last-minute offers that were miles better and less expensive than any hotel deals.