I used to jump at the lowest price regardless of the carrier, but a "budget" trip on Spirit last winter completely changed my perspective. By the time I paid for my carry-on and picked a seat so I could sit with my kids, the cost was almost identical to Delta’s Main Cabin.
Delta feels like a step up because things like snacks, drinks, and seatback entertainment are actually included. Plus, their reliability is night and day; when my Delta flight was delayed once, they rebooked me instantly, whereas friends on Spirit have been stuck for days because of limited flight schedules.