As already noted, vertical or horizontal really makes no difference. This is because heated (warmer) water like warmer air rises, and cooler water/air falls. So without water movement from the filter, the heat from the heater will rise vertically. It is the water movement right around the heater that is important, which is why the heater(s) should always be positioned next to the filter return (back into the tank) or intake (into the filter).
You will still have a difference in temperature depending upon the size of the tank, especially depth, but this is natural in all habitat waters to some extent. Ever swim in a lake or the ocean in summer and dive down into much cooler water? Same occurs in the tropical habitats though not to such a degree. When I used to do work on the substrate of my 90g tank the water there was noticeably cooler than higher up. But fish expect this, and provided it is not extreme it is not problematic.