|MadSci Network: Botany|
In most cases, tap water is fine if you have let it stand for a few days, which allows the chlorine to evaporate. Bottled water could become costly. Now, if you are working with plants that require special growing conditions then you may want to look at your water supply (whether bottled or tap) more carefully.
Read the instructions on how to grow your plants. That will tell you whether you need water of a specific quality. The quality of tap water varies by area, but you local municipal water treatment plant ought to be able to advise you of the local water chemistry (such as acidity or hardness).
If the purpose of your experiment is to actually determine which is better, then it is pointless even reading this. Your project will give you the answer.
Try the links in the MadSci Library for more information on Botany.