Hmmmm...
Don't get me wrong, I like T5's ... but they are a tool to achieve an end, not a end in themselves. So why bother even debate about what's better? You might as well debate whether an apple is better or an orange is better. The bottom line is an apple is better to make apple juice and an orange is better to make orange juice.
I think Myka hit the nail right on the head. T5's are great, but they aren't "really madly truly" an economical alternative to halides. The lamps are cheaper but you have more of them and you need to replace them more often. I doubt that you really even get substantially less heat overall (it might be better directed though).
To me the choice for lighting should be about the size and shape of the area you wish to cover, and basically what floats your boat better.
I like both halides and T5's. I have tanks with both.
I wouldn't say one really has an edge out over the other. I like the slimline low profile the T5 fixture I use has, but I don't like that in 3 months of use that I can perceive that the lights are already dimmer. I like how my halides last 12 to 18 months before I perceive a shift, but I don't like that I have to spend $80 to $120 to replace them when I DO have to replace them.
So in short, I think T5's both rule and suck, and I think halides both rule and suck.