I have that Samsonite bag as well, and it’s readily available on Amazon along with the smaller version. Nice, roomy bag, and it fits under the middle seat of a typical 737 or A320. A couple of caveats, though.
First, I’ve found that most wheeled underseat bags are fine for the middle and window seat, not so much for the aisle. But even then, be prepared to give up what little legroom that already exists. If you’re tall, this could get pretty uncomfortable on a long flight.
The electronics box under some seats limits space as well, so if you can, better to check where you’re sitting if possible in advance. I primarily fly Southwest and haven’t found this to be an issue so far, but it is on some other airlines.
There’s a guide on TheCarryonGuy.com that lists the size limits for underseat pet carrier storage that’s a pretty good indication of what you can use as an underseat bag.
My strategy for a long time has been to use a wheeled carryon for the overhead and a small to medium sized backpack for the “personal item”, preferably with a trolley strap that fits over the handle of the roller bag. With a backpack or tote, you can “squish” it to fit more comfortably under the seat and preserve a little more legroom. Personally, I don’t like to get stuff out of the overhead during flight, so stow what I’ll need in the backpack.
I do recommend that Samsonite bag, though. It’s big enough to pack a few days worth of clothes and depending on your seat availability, is a good choice for underseat use. However, to be safe, if you want to make sure you’ll be able to use it under the seat, I’d opt for the slightly smaller model.