Google's algorithm works better at finding really weird things deep in the bowels of the web (for example: obscure programming questions around GPU shaders in a specific framework or graphics pipeline) and when searching for local things if outside the US (because Google has the notion of Region when returning search results, so for example here in Portugal if I search in portuguese for a store to buy something I don't get results from Brasil) whilst duckduckgo works better for everything else.

Personally I default to duckduckgo and only use Google when duckduckgo isn't returning good enough results, which is surprisingly unusual.