We considered both the AT Atlas and the FWC Project M, to go on a Silverado/Sierra with 6 1/2 foot bed, and decided to go with an AT Atlas. Then we were fortunate to find a used AT Atlas for sale so now we are waiting for a truck.
The AT Atlas is considerably narrow and lower than the Project M, which was a major, perhaps deciding, factor in our decision since we plan to leave the unit on the truck full time, and will be using the truck for carrying stuff and towing boats. The AT Atlas fits behind the truck cab without protruding. Visibility with AT Atlas will be the same as with a regular cap/topper. The Project M would protrude out the sides and make backing with our boats more difficult. The AT Atlas is also lower with the top down than the Project M. The combination of not protruding at the sides and the lower height should result in better fuel economy with the AT Atlas. Space inside with the top down will be as better than a regular cab height cab height cap/topper due to an additional 4" or so of height from the truck bed floor.
The AT Atlas's (sleeping) bed is narrower at 48" but more height (about 36") from the bed to the ceiling with the top up. After we stretched out together in an AT Atlas we decided the bed width was acceptable. Some couples would find the 48" width too narrow.
Build quality of the AT Atlas is excellent. We were able to compare it to several new FWC units (but not a Project M) and we preferred the construction of the AT Atlas.
For maximizing interior room with the top up and bed size the Project M would be preferred.