A Rev Tent is a Rev Tent is a Rev Tent.   

The sizes are all the same.  The Rev Tent is the same tent whether you use in on the ground, in the bed of a pick-up or on the roof of a vehicle.   

All Rev Tents are the same except for fabric and wether or not they are sold bundled with accessories.  

The different names either refer to 1) the color or performance of the fabric.  For example, the Scout is green and gray, the Desert is orange and tan, the Ember Tent has black-out fabric), or 2)  the primary use of the tent.  The Rev Rack Tent is a Rev Tent that comes with a mounting solution to use on platform style roof racks.  The Rev Roof Top Tent is a Rev Tent that comes with a platform and ladder to sleep on vehicle with cross bars or smaller roof racks.  

You just need to choose the color and performance of the tent fabric,  and the position you want to use the tent in when sleeping.