Which is warmer for harsh winters?

The Nuuk Parka. Its 250 g/m² synthetic fill and thigh-length cut hold more warmth than the Canyon Shield Parka's lighter padding.

Which has the better hood?

The Nuuk's hood is fixed and fleece-lined for storm protection. The Canyon Shield's hood detaches, which suits milder days or wearing a hat instead.

Is the Nuuk too heavy for daily wear?

At 1820 g it is heavy. It suits cold commutes and outdoor days. For city wear in milder winters, the Canyon Shield is easier to move in.

Do both keep rain out?

Both list a 10,000 mm water column, so both handle steady rain. Neither is a shell for prolonged downpours.

Is the Canyon Shield worth stepping up to over the Nuuk?

Only if you value the detachable hood and lighter weight. The Nuuk offers more insulation, length and pockets for the same kind of winter use.

Fjällräven Nuuk Parka vs Jack Wolfskin Canyon Shield Parka

The Nuuk Parka is the warmer, more feature-loaded coat, with 250 g/m² synthetic fill, a thigh-length cut, 11 pockets and a storm hood. The Canyon Shield Parka is lighter and simpler, with a detachable hood and a two-way zip. Owners rate the Nuuk highly (4.8 from 62 ratings) and call it warm with room for layering, though heavy. The Jack Wolfskin is the easier everyday pick if you want a hood you can remove.

Choose the Nuuk Parka if you want maximum warmth, a long thigh-length cut and pockets for everything.
Choose the Canyon Shield Parka if you want a lighter winter coat with a detachable hood for everyday wear.

Prices

Fjällräven Nuuk Parka: $374.50 at eBay (last seen Nov 4)
Jack Wolfskin Canyon Shield Parka: $267.64 at Scandinavian Outdoor (checked today)
Difference: the Jack Wolfskin Canyon Shield Parka is $106.86 cheaper than the Fjällräven Nuuk Parka at the latest prices (Nov 4 and Nov 9)