Welcome to AA!
Are you running saltwater or freshwater?
T5 bulbs are popular because of their small diameter. You can fit more T5s in a given space. Less restrike occurs, putting more light into the tank. The smaller diameter allows good parabolic reflectors to be used for each bulb.
T8 bulbs are actually a little more energy efficient, but I imagine it's a wash when considering the extra light a T5 can put into a tank.
If the lighting in your hood is beyond hope, look at getting a retrofit kit. It's really up to you whether you want to go T5 or T8. I believe there are more bulb options with T8 for now. If you want to do a planted tank (FW
), or a reef tank (SW
), look at T5HO.