We use Ubuntu because our cloud provider supports it and doesn't support Debian (though we could probably hassle them into doing so if we really wanted to push it).
We are in fact leaving them by the end of 2014, er the end of 2015, er some time in 2016. (It turns out disentangling years of encrusted dependencies is complicated.)
But in practice, Ubuntu is a perfectly good version of Debian for our purposes, so we'll probably stick with it. The devs also like it on their desktops so feel happier targeting the same thing on the server.