The T5's are completely different from the T8's because they have an electric ballast and the electric ballast has a higher, quicker, constancy light projection. Or to put it another way when you use the old T8 fixtures which use a magnetic ballast they don't cycle as fast and the light actually has a slight flicker that takes place. This flickering, though unnoticeable to our eyes does affect the plant growth. I can get more technical, but this is the basic difference, besides the T5's have higher lumens, and less heat, but yes indeed do cost more. But don't get me wrong, people have put together some really nice T8 set ups and had good success with them and, what a T8 4 bulb fixture runs for about $50? Some people love that facter and hook it up without any regrets. So to each his own.
Adieu