We did eastern before western. Getting to Mexico is easier than getting deeper into the Caribbean for us, and St Maarten (at the time a frequent stop) was more intriguing at the time than Jamaica as my husband wanted to go to the beach with 747s landing (that has since stopped).
We always pick based on what we want to do. We are going Eastern again on the Treasure because we haven't been to Tortola yet, and my husband really wants to go to Callwood. The extra sea day to explore the ship further is a bonus - many may prefer another port day, but I would rather be onboard for my first sailing (possibly only, still not interested in sailing on the Wish again).