Also, the t5 ho tubes mean you need less of them, so getting more of the usable light out of them into the tank is easier. If you have to put in so many tubes that they are virtually touching, then a lot of light will just hit the next tube over and be absorbed by it, instead of hitting the reflector and then the tank.
Also, T5's are slightly shorter so when you make your hood without thinking about how the lights going to fit, they will fit whereas a t8 will not. t5 ho batten fittings are a lot more expensive then a cheap t8 one, but comparable to a decent t8, so you gain in the lower number of fittings you need unless you like buying the cheapest thorn piece of crap that there is.