I believe Spirit is better for network connectivity.

If I choose a route like PHL-MSY, I see that Frontier offers less than daily service in March.
For example,
3/14 0 nonstop flights
3/15 1 nonstop flight
3/16 0 nonstop flights
3/17 1 nonstop flight

On the days, it has a nonstop, it's the only option available.

Spirit is less than daily but every day there are at least 2 1-stops.

For example,
3/16 1 nonstop flight available that day, but there are 2 1-stop flight (connect in FLL) available as a fallback in case flight is cancelled or you miss your flight.

There was once a time when I couldn't clear security on time and missed a DFW-PHL nonstop. The Spirit agent put me on a DFW-ATL-PHL ticket that left a little later at no charge.

I've noticed with Frontier, it's all or nothing, or some really out of the way connect in DEN option, but Spirit has connections typically available as well at PHL.