It is never a bad idea to pre-book, especially when you know you want to do that excursion. I didn't recall any people, other than the cruiseline excursion on the pier by the ship. I do believe too, that it would pretty much be the same price, but with the excursion, they are providing the bus over to Atlantis and the return trip. You can catch the bus back anytime you want, you can wonder around a good portion of the property, relax at the beach, gamble in their casino, etc.