Best choices are metal halides or T5s (or if you have a lot of money to spend, LEDs)
Metal Halide
Pros:
Average quality fixture $600-700
Great light output/PAR
You can keep anything in the tank (SPS, LPS, nems, clams, etc.)
Shimmer effect (the way the light ripples in the tank)
Cons:
High running costs
High heat output
Expensive bulbs
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T5:
Pros:
Average quality fixture $300
Little heat output
Low running costs
A quality fixture (Current USA, TEK) will allow you to keep almost anything.
Cons:
No shimmer effect
Bulbs aren't as expensive as MH (if you get a 6 lamp fixture, replacement can be close to MH if you get quality bulbs such as Geismann or ATI)
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I don't have a fixture yet, but when my tax money comes in I'm going to get a Current USA T5 Nova Extreme Pro which has six 54w bulbs with individual reflectors. I'm choosing the T5 because MH have been known to heat up tanks (especially of our 55g size), so a chiller would most likely be needed. I also like the T5s because I can keep almost anything in the tank with the 6 lamps individually reflected.
If you go with any other T5, make sure the bulbs have their own individual reflectors; this makes them much more efficient by directing more light into the tank. Changing the bulbs to a higher quality (Geismann/ATI) will also create noticeably better lighting. You can get away with having a 4bulb fixture, however you will have a difficult time supporting high light corals.
MH is a great lighting source if you can afford it. It's best used in deep reef aquariums (water line to sandbed is >24") since you'd get better light penetration; or for people wanting to keep SPS corals on a low shelf in a shallow tank. If you don't want any limitations on what you can add to your aquarium, go with MH.
*A good light will go a long way, but nothing will survive in poor quality water.