Was going to do a thread about this (might still as well). My situation is similar...it was an F9/NK mix into 2017, but since the start of 2018 and getting elite status, it's all F9 (except Asia/South America flights or cases where I spent existing miles). So if the goal is to promote loyalty, it's working.

I've been destination-flexible to an extent (find the rock-bottom deal and then find the reason to go). Spirit has a slight edge out of ATL on destinations, especially if you factor in international through the FLL hub...I've taken them to LIM, SJO, CTG, MDE, etc. Lately, though, their deep-discounted awards for cardholders are often scarce, and if the ATL-FLL feeder route is heavily booked then that can hurt availability as well. They also just keep adding airline-imposed fees or inventing new ones, effectively raising the "floor" they want for paid and award tickets even where the "fare" is $0. Though I s'pose the deteriorating situation on award availability to plum destinations at low numbers of miles was inevitable as they added routes and new markets but were far slower about adding capacity internationally.

They also have elites really in name only, not being willing to sacrifice potential ancillary-fee revenue for loyalty. And a greater percentage of the FAs - even some young ones - get weatherbeaten too soon and fall into a jail warden/drill-sergeant mentality with the pax. The attitude is usually a little better on Frontier.

Frontier doesn't have as many routes out of ATL and because it's distance based and a deep-sale RT to a western destination in shoulder or off-season might be only $20 more than a RT to, say, Florida or PHL, I have a bias toward the western destinations that's reminiscent of a similar bias for all those years flying the legacies (even routes like ATL-EWR-LAS and v.v. on CO).

I tend to bottom-fish and try to zig when others zag, which means not many bookings in the Thanksgiving/Christmas high season or for about 7-8 weeks in summer.

As for perks, the full carry-on is a big improvement over Spirit's rucksack. Stretch seats aren't as good as a Big Front Seat, but Spirit wants extra money for those and has roped them off from perks, making it no contest. The regular coach seats are cramped on both airlines, but the presentation looks slightly better on F9. Drink prices are similar but Frontier's free water says a lot.

With status and a good batting average on the stretch-seat upgrades, Frontier actually compares favorably to the coach experience on legacies these days, with the non-complimentary soft drink the most noticeable difference. Spirit, OTOH, remains solidly with the ULCC experience and you don't get much of a break by flying them a lot.