A couple of things. Mostly, aesthetics. I like the shape of the Atlas. Project M, OVRLND and a few others are just too squared off in my opinion. Project M would be my second choice. I do like a lot of things about it. Another big reason to go with either ATO or 4WC is company history/organization, size and lead time. They are bigger and older than any other shell pop top builder and by far the quickest build option right now as far as I can tell. They have been at it a long time and seem to have a better developed process and vendor network. I'm actually not sure ATO and 4WC are the same company but I think they are. Perhaps someone can answer that definitively?
I have had a couple of wedge type pop up campers, most recently an Ursa Minor pop top on a Rubicon. These are just too tight in my opinion. We always felt very cramped and not just because the Jeep was small. For this reason I'm not interested in either of the wedge options from ATO.
If I could have whatever I wanted today I would probably choose a Khaya but they are way out of my budget. So much so that I didn't even mention them for consideration. Even though they are a wedge they have lots of space since the bed folds up on struts. I've been in a couple and they are really roomy feeling.