Okay well what lighting system you research and ultimately select is largely dependant on what type of corals you want to keep. Flame scallops are not dependant on light for survival so you don't need to worry about them. They have a very low surival rate, so as far as lighting is concerned, they are equal-opportunity-diers.
Moonlights are used to allow the aquarist to view the animals at night. This can be a really neat thing... it's often said that reef tanks are even more interesting at night because of all the neat things that come out... but they have a marginal, if any, benefit to the animals. They are an accessory or option on lighting systems that should be secondary to your actual selection.
The 3 systems you mentioned... T5, Halide (MH), or Power Compact (PC):
PC is an older fluorescent technology. It's advantages are that it's inexpensive and the bulbs are readily available. It's main disadvantage is that if you want to keep very light hungry livestock, such as stony corals, giant clams, or anemones, you usually can't fit enough bulbs over the top of the tank to adequately light it. It's a great beginner technology and will allow you to keep everything except what I listed above.
MH is the Cadillac of sorts. It's advantages are that it's incredibly intense, so it penetrates into deep tanks (it's really the only option for tanks deeper than 18"), it's the most efficient lighting currently available, and it will allow you to keep anything you want. It's disadvantages are that they run very hot, sometimes requiring the tank they are lighting to have a chiller, and it's very expensive. It's pinpoint source also gives you the shimmer effect, which some people like and some don't. I have halide on my reef systems and enjoy the shimmer.
T5 is a new player out there that has many of the advantages of halide, but is not quite as intense. Generally, with a normal shape aquarium, you can keep whatever you want under these also (though I still debate those who say stony corals will do well under them). They are priced near halides, but not quite as expensive, and don't have the heat problems. The biggest disadvantage is that they are not fully penetrated into the market yet, so finding bulbs and such is sometimes difficult, especially if you accidentally break one and are in a hurry to get a replacement locally (so keep a spare bulb or two on hand). The only other disadvantage is that because it's a fluorescent technology, you need a large area for the bulbs, so you don't have pendant options for them.
hope this helps...
