Underseat Pro was designed to be under seat complaint with Spirit and Frontier. The bag includes a trolley sleeve, wheels and a shoulder strap. My last AA flag, the gate agent did make a few people put the personal item in the sizer.
Take Off luggage is hard sided without a trolley sleeve to be under seat compliant with Spirit and Frontier. This one requires the wheels to be removed for the sizer. My last Spirit flag, the gate agent recognized the brand and said you are good.