Actually, they all seem to me pretty limited compared to the answers you get from Wolfram Alpha.
Also c.f. the responses to one of their test questions, "how much is a quarter cup of butter?". Google makes fun of the inquiry. Wolfram Alpha gives you a thorough nutritional profile, and links to variations based on international cup sizing and different types of butter.
In the article, the question "How old is the Lincoln Tunnel" struck me as incorrectly formatted for the parser (I know, that's the point), so I asked Siri, "When was the Lincoln Tunnel built." The Wikipedia article on the Lincoln Tunnel was returned. Wolfram Alpha was listed under other sources, so I chose that. The response? "1937"
Luckily if you say "Wolfram XXX" instead of just "XXX" Siri will route your question straight to Alpha no-questions-asked.
The result Google also gives you is wrong (since its also a just a snippet).