I second the recommendation to do SXM first, then SBH. At least that's what we did our first trip, and loved it. The poster above's reasoning makes sense.

Our favorite place to stay is Tom Beach on St. Jean beach. You're not too far from anywhere on the island. Definitely well worth it to rent a car.

All the beaches are fantastic. C/O was very common primarily toward the ends of Saline and Gouverneur.