I have no clue how selective a state SC summer position is versus a federal district court. But, I do know that most large firms will view a federal district court position more highly than state SC. First, state SC is obviously state specific, so the federal position is much more portable. Second, a given ettorney looking at your resume more than likely has never been in front of the state SC, whereas the interview more than likely has been to federal court if s/he is a litigator. The skills you learn in federal court are much more applicable to life as an entry level associate than in a state SC position. I would also venture to say that it is more interesting.
In my opinion the ranking is as follows:
1) Large law firm paying Summer Assocaite
2) Federal District Court Summer Clerk
3) State Supreme Court Summer Clerk
4) Small firm unpaid internship