I got the idea that Nemo is kind of considered an experimental field for introducing new ideas, while Sailfish isn't. However this goes somewhat beyond just the UI itself. There are a number of key applications (ranging from e-mail cleint and browser to IM client and etc.) which can be considered a requirement for any usable modern OS. I'd rather prefer to see some synergy in these (between Nemo and Sailfish), and of course them being fully open. But so far it's not the case.