My perfect hybris allows me to take on a discussion with a gate agent any time. At the latest when I explain that the duffel has a smaller foot print but more volume and will actually use the overhead space better, I win.

I have not been challenged yet by a GA on a duffel. I have made the test of tests with a fully stuffed VD25 that measures 25x12x12.

There are two or three factors that make duffels very good for carry-on:

1. They look smaller. It's a visual thing. They simply look more compact.
2. Everybody uses them as small carry bags. They are associated with hand carry and light weight, not with heavy traveling utensils that need wheels or backpack straps. In this sense, they are on the opposite side of backpacks. Backpacks ring an alarm bell, duffels fly under the radar.
3. This is especially true for models without wheels. They can be easily compressed to fit the sizer.

A duffel of the dimensions I mentioned above will need just a little squeeze in one direction but it looks (even in the sizer) much smaller than max size in the two other directions.

So, on principle, duffels are the most space efficient carry-on solution. If wheels are desired, there are plenty of wheeled models available or a cart can be used.

Till