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