To be honest, I would have to go with ceiling mount, because wall mount alarms can easily be covered up, and may not cover a lot of space. With ceiling mount alarms, they can cover much more space. A person walking down the main aisle of Walmart may not see a wall mount strobe down one of the side aisles by the fire exit, compared to ceiling mount devices, they can see it flashing, even if they are down a few aisles from it. In addition, if the device is defective, they can look and see other devices are flashing in other parts of the store.
Not to say I don’t like wall mount devices. In smaller buildings, in my opinion, wall mount devices are more appropriate.