Having flown both with a pretty well-stuffed backpack:

- Neither cared about putting items in the overhead bin. On both, announcements were made about putting items under seats, and I did see FAs tell folks to wait until boarding was complete if they wanted to put personal items in the overhead bins on Frontier. No one stopped me on either when I put my bag in the overhead after the last few pax boarded.

- Spirit, interestingly enough, was most strict at the point of check-in (they had an agent assigned to size everyone's bags and advise them to pay the carry-on fee at check-in as it would be lower than at the gate). The rule of thumb seemed to be if you could easily squeeze it into the sizer, they'd let it go. I've also seen this happen at BWI and LAX with them.

- Frontier was strictest at the gate; I saw several passengers get "sized" though none who wound up having to pay the fee. Of note, I only saw them do this for folks carrying bags by hand; no one with (even large) backpacks seemed to be stopped. At RDU and DEN enforcement seemed similarly lax.