Your wife is a US citizen so if you have to file taxes in the US because of it, she would too.
To find out about retirement, benefits etc., maybe try a tax lawyer? You could ask at the same time if there is an obligation to file taxes for US citizens. According to the US embassy in Germany, I guess you do: http://germany.usembassy.gov/germany/faqs/tax.html#8
The benefits of dual is that you don't need to go through another sponsorship with added cost and waiting time. If you get US citizenship and you decide to move back to the US, you just do it. I don't know if the US has the same law as Canada that your PR is protected if you are living with a US citizen spouse but your wife after she gets her Canada PR would not need to worry about Canadian citizenship because as long as she's living with you, no matter where you are, her PR would be protected as the spouse of a Canadian.