Hi,
I can not tell you about the Atlantis sub, but I have done the S&SIT. The basic tour is from the bus with a lot of scenery that passes you by. There are two stops before the Seaworld Semi-sub tour. The driver makes the choice of Cole Bay or Orient Bay, this is a ten minute stop for photos and then on to the open air market at Marigot. With good weather, this is fun and colorful. We did drive by the most beautiful of photo opportunites which was at Orient Bay. I was disappointed in that fact. Our driver was not interested in his guests....he spent most of his time on his cell phone. I would have to say that the semi-sub is probably a better choice of the two for the little ones only for the fact that if they become frightened down below, you may return via the steps to the top deck. If I recall, the total sub tour was thirty to forty minutes. I really believe the tour is only as good as the tour guide. I sure hope you get a good one. I would recommend the Butterfly Farm tour....very interesting and more on their level. The island tour was much better for us on this excursion. We made a stop at Orient Bay.....breath taking. Good luck with your choice.