Ok. the first thing you need to keep in mind is the size and depth of your aquarium. and ill explain, the depth determines how low the light will reach. so if you have a very tall aquarium, you will need higher intensity lighting in order to penetrate deeper. the second thing you need to think about is what type of reef inhabitants you will be keeping. some corals need more light than others. clams also require high intensity lighting.
Metal halides look like regular screw in light bulbs. and thats about it, they are a completely different animal. they produce a very high amount of light and very intense. They are hung like a pendant above the aquarium. the Pros about them arethey are very powerful and will enable you to keep any type of light demanding coral. the cons: they are Very expensive($300-1000), they generate alot of heat, therefore require a chiller (which also costs about $500-900) and the bulbs are very expensive to replace.
Power compacts (PC's) are similar to flourescents, however they are dual tubes, and the tubes are thinner. they produce higher intensity lighting than flourescent and are much much cheaper than MH (metal halides). the pros: they are cheaper, the bulbs are cheaper, you have many different lighting options as far as colors, they can be retrofitted into a regular hood.
VHO's (Very High Output) lights are similar to flourescents but smaller in diameter. they are single tubes, unlike PC's. the advantage of having smaller diameter tubes is the gas will run hotter, creating a higher intensity light than flourescents.
I Personally believe for most applications PC's work Wonderfully. as a matter of fact, my lfs has been running Flourescents in his coral tanks and i have bought almost all my corals from him and they all flourish in my house as well as in his displays.
all in all, it dep[ends on what you will be keeping and your tank size. i have been running PC's for the past 6 months and they work perfectly. what i did myself to save money is to buy a PC retrofit kit and build a hood myself. it has saved a heap of money. and in this hobby, every penny helps.
the best way to learn more is go on different sites and take a look at the corals, you will learn which require certain lighting, water flow, placement, and care.