We have done both Eastern and Western cruises.
We don't shop (much at least). The water is the focus of our excursions.
Snorkeling is better on the Western (or at least more opportunities): Key West, Grand Cayman and Cozamel. On the Eastern St. Maarten seems to be iffy because of the winds that kick up a lot of sand in the water. We snorkeled in St. Johns and it was very, very good - definatly will do that again - Trunk Bay way cool.
We enjoyed Orient Beach on St. Maarten a lot, very beatiful beach with lots to do. There are some very nice beaches on Grand Cayman (7 mile beach) and on Cozamel - take your pick they are all nice and about the same.
Western: something different each day (key west, sea, grand cayman, cozamel, sea,
castaway cay, home). Eastern: sea, sea, st. maarten, st. thomas, sea, castaway cay, home).
I think I liked the eastern better, and I really can't tell you why
We are going on the eastern again next year on one of the "trial" iteneraries. Taking the kids out of school for a week - oh well.
As mentioned earlier, there are more stop on the western cruise and the fees add up. And the Magic has the best parking spots!! Which probably cost more. On the eastern, there are not a lot of options on where to park in the ports they stop at.