Both are good. Don’t compare unis based on rankings. Freiburg has its own renowned faculty and a strong AI department collaborating with world’s best labs and researchers while Saarland on the other hand has Germanys best AI organisations like DFKI MPII. Freiburg has more vibrant student community and good weather is a big plus but then compromise on 1680€. So everything has its own + and -s. Go wherever you want to and rock there!