I live in South Africa and have researched Roof Top Tents a lot!
What I came up with was this:
It should preferably have a marine ply floor.
It should have mosquito netting in front of all windows and doors.
It should have a separate fly sheet (Rain cover) that sits away from the tent roof to avoid condensation inside the tent. If it has a bit of netting in the roof below the fly sheet it is even better.
The dust cover should be well made and easy to take off. If you can leave it attached to one side while the tent is open, it is ideal. Otherwise you have to find a place to put it when the tent is up.
Mattresses you can change when you feel like it.
Hannibal gets made in South Africa and is one of the 3 most popular tents around. The others are Howling Moon and Eezi Awn. They are all three excellent products. The ARB is similar, but because it is from Australia, it's more expensive over here and not so popular.
This may not really help your decision, but I would suggest choosing the cheapest one of the two.