I've done the 3 hour Circle Line several times, and there's a lot that you go by that lacks a certain glamor, and the entertainment value of this varies based on the ability of your tour guide at being a raconteur (sp?). But some of the views when you get around upper Manhattan are breathtaking, and the overview of the city is fascinating even if there is some down time here and there. And during the occasional down time during the trip, it's just a lovely, peaceful ride. Cilla, i think you'd actually enjoy the 3-hour tour at some point when you're not trying to cram in as much active fun stuff as possible.
But the 2 hour tour sounds like it's lots of the points everyone wants to see when they come to NYC and may have a limited time for touring.
OK, I wasn't any help on the question you asked. But it was a post!