Now lighting efficiency is a different story. The amount of useable light directed towards a subject is more what you are interested in.
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Supposedly T5's are popular because they are very small tube lights that when combined with INDIVIDUAL REFLECTORS for each tube, direct almost 100% of its light in to the water. So there is no loss of light, hence more lighting efficiency.

The debate as to which is better (Metal Halides, T5's, PC, ect...) rages on . A couple of things are very well known fact though. Metal Halides produce a very high amount of heat, making it almost a certainty that a chiller will be needed.
Metal Halides consume a lot of power in both their consumption and the consumption of the items it takes to control the temperatures of both the tank and the surrounding environment. They are also generally less efficient then the T5's, however they produce large amounts of light. The bulbs are more expensive for the MH's as compared to both T5's and PC's.

Guess the thing is to do research and find out which suits your needs better. I have seen people here and other places swear by both and say the other is an inferior product. It's a choice like anything else in this hobby. Best thing to do is jump on the computer and do your research.