If you are going planted, then T5HO is the way to go .... However, depending on what plants you want, and how high tech you want to go (ie CO2, fancy substrate, fertilizers...) as to how many of the light you need.
A single 36" T5HO (that would be 39W) is good for low light setup, you can have 4 or more of those if you are going super-high tech.