One thing to consider is that the Delta price doesn't include checked baggage fees. Delta charges $15 for the first checked bag (per person) and $25 for a second checked bag each way. So one checked bag each way adds another $30 to the cost of the flight and two checked bags bumps it by $80. Southwest allows two checked bags per passenger at no charge.
A second point of difference will be that Delta offers assigned seating, whereas Southwest uses "open seating." That means with Delta you will have specific seat (or seats) that is yours and it will be there whenever you board (unless Delta scrambles or deletes the assignments, which they have been known to do when they tweak their schedules a few times per year). With Southwest, you can check in online starting 24 hours in advance of the first leg of your flight and you get a letter/number combinationA, B or C and 1-60based on when you check in. Then, passengers board in order based on their letter/number combination starting with A1. Once on board, you select whatever seat you want from what isn't already occupied.
A third point of difference is that Delta can and does change its flight schedules between when you book and when you fly. If you book a year in advance, they may change the schedule three or four times before you fly. Sometimes the changes are moving a departure time ahead or back a few minutes, but sometimes they are more substantial. With Delta and Northwest merging their schedules, it's likely that Delta will have more substantial schedule changes in the coming months. Southwest does not change their schedules after they release their flights for booking. Your flight on SWA will be the same on the day you fly as on the date you book.
Finally, Delta is a participating airline in the Resort Airline Check-in service for guests at Disney-owned resorts. That allows you to check bags and get boarding passes at your resort on your day of departure. Southwest does not participate, so you need to get your bags to the airport with you. The front desk at all Disney resorts and the guest services locations in the parks and at DTD will help you with your online check in for Southwest, however.