Hi Karen, what I like best about booking the SE w/the ship is the guarantee that they will return you to the ship on time. You most likely will save some $ if you book private and most likely will be back on time but for me the peace of mind knowing I won't have to fork out more $ to get myself back to the ship if the tour should run late is worth the few (or sometimes lots) of extra $ spent.
Sorry I don't have any specific advice regarding St. P for you.