When I start to design a wetlands I always ask to myself "horizontal or vertical ? or hybrid?". Well, is not difficult to answer! I take into account the volume, the number of people treated, the available surface, energy input, and so on.
Usually I start with the horizontal, easy to be design, easy to be realized, no energy needed, and a lot of advantages.
If I have more than 50, 100 .. 450 cubic meters a day I'm sure I do an hybrid wetland with VF before and HF after. Yes, if we are in a place where the surface is free, and where there are no other problem I do an HF, sure. But usually for large volumes is better the hybrid wetland. As noted in the Vymazal abstract, the chemical reactions that occour in the two systems are different. in the hybrid system we have both, and the treatment efficiency will be higher in less space!
For example you have to treat 100 m3 of municipal wastewater.
1 Hp: only horizontal:
if you build up only HF systems you need more or less 1800 square meters of treatment wetland
2 Hp: only vertical:
if you build up only VF systems you need more or less 600 square meters of treatment wetland
3 Hp: hybrid system
if you build up an hybrid wetland systems you need more or less 1200 square meters of treatment wetland
This comparison is not made for the capital cost of building and design, only to show you the difference between the design methods.
As you can see the VF surface is the lowest but usually you have to buy pumps systems and realize the tanks for the inlet pumping.
The HF have the highest surface but the lowest cost.
The hybrid is midway but be sure that will give you the best treatment performace you can reach because it combine aerobic and anaerobic environment.