If you have booked the Land/Sea package when you check into your resort, your resort key is the key to your room on the ship. However, I think for security reasons, everyone still needs to check in or go through security at the port. Perhaps, the L/S package may speed you through this process, but I am not sure.
I booked everything separately to take advantage of the AP discount for our resort stays (before and after).
Over the last few weeks, I have been reading various trip reports on the DIS. From what I have gathered, if you take the
DCL bus, you arrive with everyone else, which means you arrive to board the ship with a much larger group. If you arrive at the port earlier than when the DCL buses arrive, you are pretty much one of the first in line. This advantage allows you more time to relax, book excursions, additional Palo reservations, spa reservations, nursery time, or things you didn't reserve online. I've also read that once on board, you can go get lunch, go to the pools, without having to deal with the crowds, as most will be just getting settled themselves. So it seemed to me that with our 2 small children, that arriving early would be an advantage to us.
It was also less expensive. Hiring a shuttle was cheaper than the DCL ground transfer for our family of 5.