I live in Maine (hard winters, warm summers). I need a new roof on my
cape style home. I would prefer a metal roof over new shingles. My
question is are "screw-down" metal roofs as good as "raised-seemed"
metal roofs. Screw down's are approx. 50% cheaper than raised seemed
here in the current market. I've heard that screw-down roofs have
serious problems after a few years with screws working loose from
heat-cold expansion. If this is true, then is the answer to pay the
50% premium and go with a raised-seemed roof? Thank you!!

Buy the more expensive product.  The nails will definitely loosen and
pop up within a few years.  I've seen it.  It's not worth it, unless
you're just doing a utility shed or something.