Caribbean Airfare Question (Best option: All-inclusive airfare, or book airfare separately?)?

Question

We're going to St. Lucia in December and are wondering, is it better to book our airfare with the resort (Sandals in this case), or to try to find rates separate from the resort. We're flying from MN. From what I've found on Orbitz, Travelocity, Expedia, etc., the flight in the all-inclusive package is cheaper. Thanks for any help!