There is basically one difference from my experience between the two.
SFML - Uses C++ design.
SDL - Uses C design.
Both are good libraries and there isn't really that much of a difference between the two other then the design. So in my opinion the choice really comes down to "Does an object oriented design matter to you?". If so then I would go for SFML, if not then either one of them can work fine for you.
Here is a few quick example to demonstrate what I am talking about take a quick look at the two codebases.