I have an Aquactinic MH/t5 fixture and I recommend them to you. The quality if excellent. I don't have any experience with the t5s for other than blue support but I'd think that you should at least, consider the MH fixture with the 175-watt bulbs. I think the wattage might serve you better for coral without any limitations.
Depends on a couple of things; what you want to keep, the look you want to give your tank (personnal taste), and the almighty dollar (how much you want to spend).
Do you have a canopy? Are you only interrested on fixtures only, can you go retro? Is heat an issue you will need to worry about? Is your tank a 55 gal regular (48" long)? If so, go with a 48" fixture.
drill...The bulbs I got with the Aquactinic fixture are the Coralvue 14,000Ks. I have two Pure Blue bulbs with it. I'm confident that the t5 unit is a good one if it comes from Aquactinics. I'd just be certain that with the depth of your tank, t5 lighting has the sufficient penetration to light your tank sufficiently enough that you'd be able to keep whichever coral, clams, etc. that you'd want to anywhere in the tank that you'd want to keep them. I'm certain that you could do that with the metal halides but I'm not certain from my own experience that 195-watts (or 3.55 watts per gallon) would bring you into the range that you'd need to keep whichever coral you might want to try.