It looks like the external one has the water entering the side, along the side of the bottle (?), generating a vortex, and exiting from the center of the bottle at the bottom. And, the CO2 enters at the top in the center. So, the vortex keeps the CO2 captured in the middle, where it stays long enough to dissolve in the water around it. And, the "bottle" looks like two bottles with the bottoms cut off, and joined to make a double ended bottle with a neck at each end. Am I right? It looks like it would work very well, but I have no idea how much CO2 would dissolve, nor how much this would decrease the flow from a powerhead or pump.
The internal version looks like it works the same way, and since the photo shows it in action, you can see what a powerful vortex it generates. If it were to not dissolve any of the CO2 you would see it bubbling out the bottom.