5cheers, there are literally dozens of tour operators that offer virtually identical SRC tours for roughly the same price. Any variation in price is typically due to the number of guests and the length of the tour since you're basically sharing the costs of the charter boat w/crew with the other guests on the boat (i.e. fewer guests and a more intimate experience is going to be more expensive).
4-stop tours typically include lunch and will likely not be appropriate for a cruise ship visitor with a limited time-frame. If you do a search of the Cayman forums you will find a VERY long list of questions identical to yours. In most of those threads, one or more posters will tout any one of the many tour operators that they used since they are all top-notch and people rarely have anything but glowing reviews for any of them. The problem with this willy-nilly approach is that many of these tours will not work for your cruise ship schedule. I would urge you again to check out the cruise critic link where you can find information specific to your cruise and the tendering schedule you are going to be restricted to. Either way, and no matter who you decide to use, I'm sure you'll have a great time.