If I were to do this over again, I would probably not use soda. In my experiments, Dr. Pepper was not only the slowest at growing, but it also had only two of three seeds germinate. This is probably a small error, but next time I would use a different solution.
Water was also a slower solution. I would probably use it again though, because water was the only natural solution. It was also my control group because, normally, people grow plants by watering.
As a replacement of Dr. Pepper, I would use a solution in which the pH is between 4 and 6. I recommend this because, in my experiments, the two solutions that grew fastest were in those categories.