If you sometimes fly Southwest, then my recommendation is to book now while the fare is good. If there is a sale (they still start on Tuesdays, but some are good and others are lame), then you can change or cancel and rebook, and keep the fare difference as a Southwest credit for a future trip. In my experience, Spirit is barebones but OK if all works according to plan, but service recovery in case of a cancellation is hit-or-miss.