Why T5? Why Not T8 or T12?

Why is T5 better than a T8 or T12 setup?
The only difference I know is the size of the tube?
I only see ads for T5 like the Bad Boy T5 or Solar Flare T5 VHO.
Does the T5 put off a better spectrum?

T12 fixtures are discontinued but ive got 6 in my garage

Thanks guys

T8 means 8/8th of an inch in diameter.
T5 means 5/8th of an inch in diameter.

the T5 being that much thinner means you can stick more over a tank and length for length you are getting more wattage.

T5's are visibly brighter.

I would say go T5 every time. not necessarily for power but for choice of tubes. with T8's you really are limited to the poverty spec tubes, all seem to glow pink and a bit crappy. with T5's you can use deltec and aquamedic tubes. both of these are visibly awesome. give a proper blue light with no pinkyness. colours flouresce exponentially better than under t8's and aquamedic tubes are the cheapest around, plus it's not too hard to find deals on for them.