While I like both itineraries and have been on both several times (7 E, 4 W), I lean toward the eastern for a few reasons. First, I like the extra day at sea. These are the days when I'm truly on vacation and do very little, other than eating, drinking, reading, and swimming. I mostly hang out on deck 10 with a book. In the late afternoons, I move to deck 4 and grab one of those comfy lounge chairs on the sunny side of the ship. I also like that Sunday is a sea day. While I like Key West just fine, I find it hard to get myself up and off the ship after flying across the country on Friday and embarking on Saturday and getting the cruise going. With the fatigue of travel, the requisite sleepless night on Friday night prior to the cruise, and the over-doing it on the first night, I'm really dragging on the first full day on board. Finally, I like St. Thomas as a cruise port. There are a lot of different things to do: golf, the Atlantis Sub, anything on St. John, the various St. Thomas beaches, banana daiquiris at MountainTop, or just wandering downtown Charlotte Amalie.
So for those reasons, I'd give the eastern a slight advantage. At this point, though, we usually book the date/price rather than the itinerary. With all things being equal, I go for the eastern.