I think it is simpler then everyone says - my experience is that all non-mainline flights do valet checking and return your bag at the gate on arrival, whereas mainline gate checking checks it through to your destination.

If you're boarding a non-mainline plane then the gate agent won't offer to gate check your bag, they'll just give you a valet tag.

Also, on many commuter airlines the FDA certificates do not allow roll-aboards on board at all, even if they fit. My carry-on bag fits very nicely on a E170 and Skywest always lets me carry it on, but the Charlotte commuter planes almost never do.