I have a comment about the question you posed that remains unanswered.

Yes there is something that competes with Metal Halide and we all have it!

Sunlight!!!

A tank using any type of lighting can keep acropora and clams if it gets at least 4 hours of direct sunlight in a south facing window. A tank like this will need plenty of algae control and should not house carnivores like Wrasses. Carnivores eat the "microverts" like gramaras that eat microalgae even before hermit crabs and snails can get to it.

BTW - other lights like mercury vapor and halogen have been tried but abandoned as unsuccesful.

Mark         "Thinking outside the box."

Since my experience is only with pc lights I can only tell you what I've learned about them.  They do need to be replaced before they burn out.  I've heard they start to lose intensity some where between 1 year and 1 and 1/2 years.  I also think it could depend on how many hours they are ran each day.  I replace my pc lights after a year and a half.  I have'nt noticed a loss of intensity between the old bulbs and the new bulbs yet. 

My opinion on mh vrs. any  otherlights is that it depends on what kind of live stock you intend on keeping.  Personally I'm very happy with my pc's.  I have a nice variety of corals that do well under this lighting.  However there are animals such as sps corals, xenia, clams, etc. that require more light,such as what they can get with mh lights.

I also enjoy the simplicity with pc's.  

I hope this helps until some can answer all your questions.

Steve