When we did the land/sea from the POR, the pick up was around 12:15. The first bus filled up partially and then went over to pick people up at the AK. We boarded around 12:30 and arrived around 1:15-1:30.
If I were to do it again, I would arrange for a car service for an earlier pick up. There were a lot of people waiting and after 4 days in the parks in line, I felt like we were all trying to get a good spot all morning. The bus was very convenient and I enjoyed the video on the way over.
If you are not worried about signing up for anything, you should be fine. When we did just the cruise (3 night), we were checked in by around 12:00-12:15 and then we went on in. It was a big difference time wise, because we didn't feel like we had to rush.
Another nice thing about the bus is that you are already checked in and can walk right on board if you have the package. I don't know if you can do this if you take a car service. We turned all of our paperwork over to the girl on the bus on the way over to port. I'm not sure what you would do with the paperwork if you had a car service.