I didn't care much for St. Martin, but St, Thomas and St. John are beautiful islands with some of the most gorgeous beaches you will ever see.
On the other hand, the western gives you three stops - Key West, Cozumel and Grand Cayman.
Another thing to consider is that the eastern gives you three sea days, where the western gives you only two. If sightseeing is what you want, go with the western. But, if you're looking to enjoy the magic of the Magic, I'd go with the eastern.
Some day I hope to be rich enough to do both back-to-back.