I used Couchsurfing (CS) long before warmshowers came on the scene. I've even stayed with the mother of one of the founders! Before couchsurfing, I used something called Hospitality Exchange.
There are a couple differences between CS and warmshowers. The obvious one is that warmshowers consists mostly of bike tourists. So, the hosts have a very good idea what bike touring is all about and what kinds of things a bike tourist might want to have. This is a huge plus.
A second difference I find is that warmshowers hosts tend to be more responsive that CS hosts. Much of this has to do with warmshowers purging of members if they aren't active once a year. CS keeps people in their listings for ever, as far as I can tell. While it might take 3 or 4 warmshowers hosts to find a place to stay, it may take a dozen CS hosts before you even get a response.
CS hosts tend to be younger people with small accommodations. I've slept on floors, couches, in RVs, and, once, in a shared bed. These days, I only look on CS for people with separate rooms available and look for hosts over 30.
Warmshowers assumes people will spend one night and move on. CS hosts often bristle at guests who only want to spend one night.
When I am planning a tour, I always check warmshowers first for a place to stay. If one isn't available, I check hotels (booking.com) next. Once I find an acceptable hotel, I check to see if there are any possible CSers. CS hosts are a rare find for me and often I don't even bother checking its site.