We did a B2B Med./WBTA a couple of years ago and stayed in 6018. We like being forward where it is less crowded and not far from the elevators. However, be sure to check out the smoking areas of the ships. Last June we stayed in 8006 on the Wonder, with a verandah, right below the smoking area . During our glacier day, when we were dead calm in the water, ashes and smoke came down on us to the point we called Guest Services and they helped a little. As long as we were not moving we still had smoke. Some folks like being lower in the ships and toward the center to avoid seasickness. Others like to be close to the kids clubs, dining rooms or adult areas. Some don't want to be near the engines. If you book early enough you increase your choices for stateroom locations. Good luck and say "hello" to the Mouse for us.
PS As for the side of the ship, for the WBTA, it doesn't make much difference. If you were going to CC, then starboard might be for you. We didn't find any advantage or disadvantage with our stateroom location during the ocean crossing as far as port views. It was high enough to see a long distance, so we were happy...although, any cruise makes us happy.