Answer : Both S1 and S2 are correct
Recursive enumerable Languages are not closed under set difference operation .But good thing about Statement S1 is as they mentioned that "L2–L1 is recursive enumerable where L1 and L2 are recursive enumerable respectively."
if L1 and L2 are Recursive enumerable then it is not sufficient to say that L2 - L1 will be Recursive enumerable because it violates the closure property.but they have already mentioned that L2 - L1 is Recursive enumerable and along with that L1 and L2 are also recursive enumerable this statements would eliminate the possibilities to make this statement S1 wrong.
Reference : Recursively enumerable language
Statement S2 is also true which is "The set of all Turing machines is countable.".
Reference : Countable TM