Did you check out our sister site http://www.PortReviews.com
? There is a general information review of each port, but there are also a number of reviews from cruisers that have reviewed specific shore excursions.
As a point of clarification is one ship going to San Juan, Nassau, St. Thomas and St. Maarten and the other going to Coco Cay, St. Thomas and St. Maarten? If so, I would probably choose the ship that was going to San Juan and Nassau as opposedto just Coco Cay.
As for the specific excursions you mentioned - the ATV on St. Maarten can be quite a lot of fun, but the helmet dive was just "so-so" (JMHO)