As maintainer I won't be able to give you an unbiased response. The biggest difference is, that SFML is a C++ library.

What motivates me to work on SFML is, that we're building something separate from SDL. I don't think, we - as in the general developer world - do anyone a favor in consolidating everything into one library (why use X, when Y does the same?). Additionally, I think it's a very interesting "field" of trying to provide a common API across different OS; and sadly it seems to be an area where few resources exist and fewer developer actually understand it, as "everyone" just uses SDL. Finally, I really like the open and welcoming community we've built over the years and at the same time, I'm happy we're not getting as much attention/pressure as SDL.