What indicates it's a prequel:
1. The teaser shows us a completly normal girl, and she holds a Big Daddy-doll (who would do that AFTER the catastrophe?). Id est, she hasn't become a Little Sister yet
2. It sounds like the 50's
3. The name is "Sea of Dreams". Rapture was Ryan's dream.
4. Rapture is rising from the sands, which tells us that the city is under constuction or already constructed
What indicates it's a sequel:
1. The girl could be a girl from the good-ending, who mourns her loss
2. "Sea of Dreams" maybe indicates that someone built another city (someone...like Mr. Ryan who perhaps survived after all?)
3. The bad ending shows some splicers who comes up to the surface
4. The logotype for BioShock 2 is more destroyed than the BioShock 1 logotype