Instructions
Use each of the games for a week each making sure not to overlap weeks. Each game gives you a report of how you are doing from day to day and allows you to review your status at any point. You can check how much weight you have lost, how much weight you have gained, or how much tone you have in comparison to when you started.
At the end of the first week, calculate how much you have lost total. Do the same thing with the second game at the end of a week using it.
Record the types of exercises offered in both games. This will allow you to compare the exercises in each game to see which one best suits your needs. You might find that the Wii Fit does not offer as good of a cardiovascular routine as the Wii Active and you are looking for a cardio-rich game; if so, then the Wii Active is probably better for you. In the same way, you might find that the Wii Active does not offer the strength training that the Wii Fit does, so if weight training is what you are looking for exclusively, the Wii Fit might be more for you.
Record the results of each game next to one another. Whichever game helped you lose the most weight will most likely be the best game for you. If you find that one of the games suits your better than the other, you may chose that game over the alternative. This is the best way to test each game because it allows you to sample both of them firsthand.