Top of the Rock or ESB?

I will be going to NYC next friday and plan on taking a double decker tour bus and you have to choose between top of the rock or the ESB. Which would you choose and WHY? Also which line would be shorter as we are on limited time so we can see a lot of the city and how much of a difference in the lines? also what time of the day would be the best? I know you can get an express pass but can you buy just the express pass since ill already have the ticket to go to the top?