reason why i said that is it will be difficult to manage both.
you will have to travel across border in order to maintain
their validity. you have to settle in canada for at least 3 yrs(continuously)
but at the same you don't have to be absent from usa for a yr (it used to be 6 months)
i'm sure how you will be doing that ( managing your job your rent and other)
The Canadian PR residency requirements are living in Canada for at least 2 years out of every 5 years.
For the US, I found this article about requirements there, http://www.visalaw.com/01jan4/12jan401.html which says that you should continue filing taxes there and keep a bank account. Back in the day, I knew some people who kept their green card for years by going to the US every 6 months but now after 9-11, I think they are watching their comings and goings more closely. Coming to the US every 6 months or every year is not guaranteed to keep you in PR status in the US anymore. If they believe you have moved away, they can still take it away. I would try to keep both and see if you can get away with it.