I've never seen anyone use T6 over their aquarium either, but have noticed it becoming more popular on the net (and ebay) for aquarium lighting. Supposedly like a T5, but cheaper. I dunno. Regardless, T5's are more powerful than PC bulbs, so the added light would be great.
To get the full benefit of T5, look for a High Output ballast/system and a system with individual reflectors vs. the single reflector. Without a HO T5 system, I'd say the effect on your tank would be minimal. If you can find a T5 HO system to add to your tank, it would increase your coral options for sure I'd imagine. T5 also penetrates deeper than PC bulbs, which is nice.
HTH