Custom fit to truck, vertical walls, custom build options, can be less expensive. Quality is excellent.
You have at least part of that backwards, which makes me wonder where you get your information from.
Another big advantage to the ovrlnd is that the bed size on a fullsize truck is much wider (approx 68") than the atlas.
I think you mean that the Project M has a wider bed size. I also think you mean that the Project M has vertical walls, because the AT Overland clearly does not.
Here is my summary:
The Project M has the vertical walls.
The Project M can be less expensive. In fact, my analysis shows that it is materially less expensive when similarly outfitted.
The Project M has a materially larger bed because the vertical walls provide a larger bed platform.
The Project M connects to the truck with included "shelves" that can be useful for storing gear or sitting on.
The Project M's shelves might also limit space for some purposes.
Thanks to the vertical walls, the Project M can be much more easily removed and re-installed.
The Project M screams "look at me, I'm going car camping and might get stuck so I have a lot of stuff hanging off the sides" (Joking).
The Project M probably has better resale value.
The AT Overland Atlas has sloped walls that limit space
AT Overland Atlas is more expensive, despite the smaller capacity.
The AT Overland has a much more sleek and stylish appearance, thanks to the sloped walls.
Due to the sloped walls, the AT Overland looks a bit goofy with the canopy brackets and those brackets are probably noisy at highway speed, but in contrast, many of the Project M designs just plain look goofy all the time regardless. Pick your poison.
Bottom line:
Project M is probably better for those wanting more space and those who don't object to the shelves protruding slightly into he camper bed area.
Project M probably better if regularly camping with two or more people who intend to sleep inside the topper.
Project M is definitely better for those that may want to remove the topper occasionally.
The AT Overland Atlas is probably better for those that want a more streamlined topper or one with a less "camper-like" appearance and those who are generally solo.
Me, I am torn between the two...