I'm always on the cusp between a size that will fit ANY carry-on bin storage/floor storage, and a size that MAY have to be checked on smaller regionals, and then something I don't CARE if it's checked or not, and something I absolutely would NEVER check because it's too nice, but that I can be SURE will fit ON BOARD regardless... so I really can't ever decide.
It's been my experience, especially recently taking a lot of regionals in and out of IAH, that these regonal jets do not always have the same overhead space, or under-seat space.
I have a very small LL Bean set that they no longer sell. Basically, a rolling Laptop case, that I modified by removing the paper dividers and laptop padding areas, and made it into an under-seat bag. This bag is basically the same size as their "Under-Seat" bags Actually, I think an inch shorter, but same width and length.. Then I added Bean's very small carry-all "accessory bag" that I know will fit anywhere (that pops over the handle of the rolling laptop). Last week, I was not able to place this rolling laptop case in the overhead of a small regional from IND to IAH, and it certainly did not fit under the seat, even though it is SMALL. I had to gate-check it. The Bean Carry-all "Accessory Bag"? Sure. but not the rolling laptop bag, small as it is.
So, unless I want to go SUPER small and basically carry a shirt, socks, underwear and some toiletries in the Bean Carry-All "Accessory Bag", I have decided there is NOTHING I can take that will guarantee I can fit it above or below seats on ALL commuter jets. That IND to IAH Jet had ridiculously small overhead bins. And the under seat storage was just big enough to fit the Bean Carry-All "Accessory" bag. Pitifully small storage areas.
But that's my experience. Mileage may vary, etc. Keep us posted!