I've had T5 in the past and quite like them. They certainly make sense in some situations. A T5 is a bit more efficient at generating light than a regular t8 fluor as you're getting the same power/light out of a smaller diameter, so it is more of a 'point source' so greater 'intensity', plus the narrow diameter means it is possible to get very efficient reflectors to work with T5's , and I tihnk these are pretty vital to efficiency. In cross section they look like 'gull wings'(like the spider reflectors inthe link). A good HO T5 is about twice as bright as a normal fluor. tube, plus as you say, you can jam them in good.
Here's a couple of things to think about lighting. First, you are using lighting to power zooanthellae algae that lives inside the body of a coral. That means, to get to the algae, the light must be powerful enough to get through some coral flesh. Forget watts per gallon, the light must be powerful enough to punch through and get to the algae. It must also be the right 'colour' when it gets there as well. Anyway, that is why I don't like watts per gallon, espeecially in the smaller size tanks. You might have a 10 gallon tank with highly coloured hard corals. Now you might put some NO fluors on top and say 'great I have 40 watts of light on my tank that really holds 8 gals , therefore I have 5 wpg, therefore I can keep all sorts of 'bright ' things. WRONG. Your light is so dispersed that it is not intense enough to do the job. Big tank, small tank, you still need a minimum level of power to break thorugh to the zooanthellae, so you would still need a 150 Watt halide, or some more powerful fluorescents. Plus the dispersed nature of fluor lighting means it doesn't puch through depth so well.
Fluor versus halide - dispersed light v. point source. I have used fluorescents (and use them recently) to grow all sorts of stuff successfully, and you can certainly get a good amount of light, measured as lumens, into your tank. Perhaps the big difference between the two is how the lumens are concentrated. With a fluor you get a constant intensity of light, but a halide is a point source and so you get a more uneven distribution with shadows, ripples and so on. Now you might think ripple lines are trivial, but a lot of evidense says no - each ripple acts as a light focusing event, incresing and decreasing the amount of light on a very short timescale, by up to a magnitude of 15. This apparently is very good for stimulating growth and colouring as this is a response to the max intensity at any time, not the average intensity. Ripple lines are good! Also the microsecond scale rest from light is also cited as being good for preventing bubble disease by giving the zooanthellae a breather.
Intensity/duration - don't think you can get away with lower intensity for longer time - it doesn't work like that. Power is what you need. If power is costy for you, it is possible to cut back to only running halides for 3 or 4 hours a day without too much bad happening. You can't run fluors 18 hours a day to try to match this.
Cost/organisation - I like halides as the long term cost is good (less bulbs to change, worry about) and it make s your wiring simpler. I have a hood with 6 overcharged NO fluors jammed into it in the basement and the wiring is horrible and messy (and potentially dangerous). Also I like open top tanks, and the ripple lines are important I think.
So if I was going to spend money for you I'd get halides. If I look at your link, that's 300 USD for a twin tube T5, and you are going to want 2 of those, compared to whatever for 2 150 DE halides.
Also I see PC is an option for you, and that's not bad but I think T5 is a bit brighter
My advice is go look at tanks with the different lights on and see what looks good to you.
