Treated cedar shakes should last a good 30 to 50 years. One thing to check is your fire code. In some locations, cedar shingles must be treated to be fire resistant. A check with your insurance company would also be a good idea.

I like both shakes and metal roofing. Another option which would seem very appropriate for your house is natural slate (very expensive) or some of the newer products that look like slate. The latter really are convincing, are warranted for a very long period and are naturally fireproof.

Yes, like Mainegrower says, be sure to check with your insurance company. Depending on the section of the country where you live, you might get a break in rates if you install a standing seam metal roof. Like here in hurricane country. Or probably out in forest fire country too.

One other thread mentioned that in snow/ice country, where slate was a nice roofing choice, they put standing seam metal on lower porch roofs where falling ice etc would tend to break the slates. So both materials would be appropriate to use on the same house simultaneously.