Spa Frog is manufactured by King Technology and as chaser already indicated can be used in combination with Bromine. The floating bromine system is an option part of the Spa Frog system-but isnt required. In fact Id recommend not using a floating dispenser of chemicals like Bromine or Chlorine since it removes the consumer from the responsibilities of maintaining proper sanitation levels and other water care.
Nature 2 is manufactured by Zodiac which also makes the Vision cartridge which Dimension One distributes as a part of their water care system. As chaser has said, Nature 2 cannot be used with Bromine...
Both products do basically the same thing- In my opinion, this is a Pepsi vs Coke debate.