I'd run with option 2. The Sinking Tower is very much a one-shot so would be a great intro for players new to the system, you could run it with the pre-gen PC's then have your group roll up their own for Secrets of the Dragon Emperor.
Just another input, I like Magistrate's Gambit as one-shot intro for players than Sinking Tower. So suggestion is sliding Magistrate's Gambit first, then Sinking tower, etc.