Have flown on Frontier several times with no issues. Relatives were flying to Vegas a couple of years ago and their departing flight was delayed, so Frontier gave them free tickets they could use within a year. They thought it was wonderful they got a future free flight, guess it depends on your impression of how things are handled.
Where I used to live, Spirit was CONSTANTLY on the local news showing irate passengers at the airport who were stuck because a flight was cancelled and no good options existed. If they don't fly daily flights on a particular route, could have major issues when things go wrong. I personally would NEVER fly on Spirit. Also make sure to check the TOTAL cost of flights when doing comparison shopping. Many only show the base ticket price to make it SEEM like it is a bargain, but once you add in the various extras, not so much.
There are risks with any of the these 'shoe-string' airlines, but I consider Frontier to be a much better choice compared to Spirit.
Have noticed recently that Frontier will sometimes show a connecting flight with an exorbitant layover time (8+hrs) instead of indicating they don't service that route. Make sure to review the connecting flight information to avoid surprises when traveling.