This is really a challenging decision. I'm just trying to get a good solid sturdy bag and not have to worry about buying another one again later. I will be spending the next few years traveling very frequently on Spirit and Frontier so I'm looking for a bag which I can place underseat but get the maximum carrying capacity. There are a ton of options which are guaranteed to fit in the sizer and ultimately under the seat, but there are a good selection of bags which MAYBE will work. They don't fit the dimensions, but there exist pictures of the bags both underseats and fitting into the sizers.
The Aeronaut 30 seems great, and large but I do occasionally travel for work and while I just typically take a tech backpack and a rollaboard on those trip, since it's reimbursed and I fly major carriers, it might be nice to go single bag on those as well so the Aeronaut 30 is not ideal for the tech I have to carry. Technically it's too big, but because of the shape, it seems like it would be easy to scrunch into the sizer.
The Western Flyer is a great option. The dimensions are just under the allowable (18 x 14 x 8) with the bag being 17.9 x 12 x 7.1. No laptop space, but could easily fit one into the two existing sections with a sling. I don't like the water pouch though, it doesn't make sense to me in such a prime piece of real estate on the bag. I guess I can use it for other stuff, but for this price, I want it to be more multi-functional. Maybe I should just start to carry a water bottle more though, could be a sign.
The Tristar is a great looking bag. It's only slightly larger than the WF but it looks much bigger and while I've seen pictures of it under seats and even in sizers, I just don't trust it. It has the same water bottle pouch as the WF, but it also has a third deeper pocket which can fit my ipad in a sleeve and a dedicated laptop section. I would assume I could probably fit a thin laptop case maybe with a simple strap so that I could take the laptop and a pen and pad to meetings.
Some other considerations. I love the burnt orange color and that's only available now in the Tristar. I would have to move down to maybe my third or fourth favorite colorway if I went with the A30 and the WF has my second favorite. My first trip comes up before the burnt orange will be available on the A30 but honestly it's just a color.
They are, not that it matters, but: 1. Burnt Orange, 2. Navy/Wasabi, 3 (tie). Verde, 4 (tie). Steel/Wasabi (I like Wasabi inside)
I think I prefer the look of the A30 and I could see myself getting that and maybe adding a brain bag or S18/25 down the line.
Should I be considering the S25 here? it misses the 18 x 14 x 8 in two dimensions, coming in at 20 x 13.4 x 9.1 but it also seems the most scrunchable of the options.
Anyway, sorry for this somewhat long winded stream of consciousness post but I have a feeling I am not the only one that has struggled with the options.