While the durability of metal roofing is one thing, there are the issues of snow sliding off it to places you'd rather it not pile up. If you are building a new house and think that you ought to have a metal roof so you can avoid having to shovel snow off it, think again. As pointed out, the roof must be designed for a heavy snow load, so for a new house there isn't anything particularly wrong with letting the snow stay there.
If the intent is to avoid ice dam formation and excessive icicle formation, bear in mind that in nearly every instance the formation of ice dams and icicles is an indication of heat leaking into the space under the roof deck. For a new house, there is no reason why this ought to happen, if care is taken to avoid this sort of heat leakage. Take care to seal the upper level ceiling against air leakage into the attic space. Avoid can lights there; they are a bad idea, from a building science point of view. Provide adequate soffit and ridge ventilation. Keep the heat out of the attic and you avoid roof ice issues.
For a lot of good information on this sort of thing, a good site to browse is
www.greenbuildingadvisor.com.