I would pick the Dawn over the Sun. For one she is about 20% bigger and holds about the same about of passengers. She is also brand new and the Flagship of the NCLfleet, so I think a little bit more upscale than the Sun. The ports of call are almost the same. You can wait until you are on board to book the tours. You'll have no problems. What Cat. cabin are you looking to booK.
Any other questions, please feel free to write.