We always fly Delta because we're Delta Medallion members. For our Aruba trips we usually don't book our flights until about 3 months prior to our trip. For our market/area that seems to always give us the best fares. Other cities/markets could be completely different. Depends on where you're flying from, the time of year, and what kinds of airline competition you have at your airport.
We typically pay anywhere from $420 - $520/person for airfare on Delta.