Well, from a very, very cursory search, it sounds like SDL uses software rendering, as opposed to the OpenGL-based hardware acceleration SFML uses, as well as omitting several useful APIs. It's also supposed to be Object-Oriented, meaning that you'll have to write cleaner code to get it to work, but you'll also run into less headaches working on it. In the world of APIs, newer tends to mean "more evolved for current usage," along with "doing things made impossible previously by physical or mechanical constraints." I wouldn't say hardware acceleration is "some way," I'd say that, if you knew how to use SDL, and could move to SFML, it would probably make your game faster, easier to code, and less likely to cause you headaches from "gotchas."
In other words, faster, simpler, and cleaner, is what I've been seeing. It's also under active development, too, so unless you have need for a large development community to help you, SFML should have sufficient advantages to make SDL the less desirable option.
