The pictures on that site suck big time, but if I had to guess I'd say you'd have to limit the size of plants with that system. You've got to factor the height off the floor against the height of your light. 20" isn't really tall, but it's taller than buckets. What kind of height are you working with (ie how high can you go with your lights?)
For the lighting I'd say the HID's would be the way to go if you've got the dough. Can you even fit T5's in your closet? With the fluorescents you can grow the plants taller, but then again the plants will definitely get taller with the weaker light. I'd say drop the cash now for the 400w HID.
In a closet where you want to keep plants small it might work best to get a HPS ballast and light, and a MH conversion bulb. Do a short veg then switch to flowering. You can't beat the density HID's will give.
Just remember, with the HIDs you're going to have to have a much larger venting system to keep the heat down. You can cook with them.
EDIT: just thought of something else - are you growing from seed or clone? The system you linked would be better for clones where you know they're all girls. It could be hard to get the boys out if you grow from seeds - their roots could get tangled. Then you'd have to cut them off at the stalk and run SensiZym or something like that to break down the roots without making your girls sick. I think you might be happier with a simpler system. Check out basic ebb and flow systems or perhaps some hempy buckets. You can't get simpler than a hempy, and it gives great results.
