T5's aren't cheaper. A 400 watt HPS with Hortilux bulb puts out 55,000 lumens. A T5 eight-bulb system puts out 40,000 lumens for 432 watts (54x8) of electricity useage. So, the HPS light puts out more lumens, for less cost on your electric bill. You can also just use a $10 bulb with the HPS light (albeit with less light output than a Hortilux bulb, but still more light than the T5). I don't know how much eight of those T5 bulbs cost, but I imagine it's more than $1.25/bulb.

No matter how you cut it, an HPS system (of equivalent wattage) generates more light, uses less electricity, has better canopy penetration, and in a pinch you can replace your bulb for ten bucks.

The big advantage of a T5 seems to be that it generates less heat than a HPS light. But if you can handle the heat of a HPS light, I don't see a lot of reason to flower with T5s, although they're probably okay for ScroG. They'd probably make nice sidelights if you hung them vertically, in conjunction with a primary HPS light. They do seem like they'd be nice for vegging, since you don't need as much light then as you do during flowering, and young plants are more sensitive to heat.

__________________
"Before you consider one an enemy, first walk a mile in his shoes. Then, if he *is* an enemy, you're a mile ahead, and you have his shoes."