shingle or metal roof?

Greetings everyone. I need to replace my existing roof and I have some questions that I hope some of you can help me out.
I have a one story house, with two layers of Asphalt shingle, around 1400 sqft. I am in the bay area. The weather here is mostly nice, but in the winter there can be quite a bit of heavy rain. My current roof is leaking and repairing cost too much, so I figured just replace the whole thing.
I've spoken with several contractors. The most recent one seems to be quite knowledgable. He mentioned two things..
1. their bid includes the polywood layer on top, before the tar paper. I thought this is a standard, no? other contractors don't mention it.
2. he said they actually replaces the flashing in the stucco, by breaking about few inches of the wall where the roof meets the wall (some sections do), and most others don't. Is this important?
The bids for shingle range from 6k to 10k. I got about 3 bids so far.
I also talked with a metal roof contractor, the metal roof is 14k. The brand is Gerard. He seems to make it sound like the best thing there is. From what I gathered, metal roof lasts longer, is lighter, may be better in earthquake (which is where I live), and he can put in a layer of insulations under it.
Do metal roof really works as well as they claim? Does the expand/contraction loses the nail seal and lets water in after a while? Any maintenance? This is Gerard stone coated and I do like the look of tiled roofs.
one important goal for me is to replace the roof only once. I don't want to go through the hassle of doing it all over again in another 20 years, when I have a family, kids, and who knows what and probably less $$$.
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