I think it goes past which light to get. What you do to your tank beyond the lights is what makes it successful. Many people claim T5's pales out their corals and made them pastel looking, while others have tanks that look gorgeous under them. Probably doesn't matter what lights you get as much as how your flow, feedings, parameters, etc are. As far as physical difference, T5 can be less heat and less energy. Some people also claim they could not get good colors with T5's and went back to MH. So it is all individual. Just take care of your tank and keep it stable and it will look good. :0)