You are correct about the Cdn $ being a bit stronger than the US $ right now, however the price of an iPhone 5 (unlocked) in Canada purchased from Apple is $699 + taxes. Sales tax varies by province, in Ontario you will pay 13% sales tax for a total of $789.87. The unlocked iPhone 5 isn't available yet in the US, however indications are that it will sell there for $649 (+ taxes). Depending on region, you could end up saving $100 or more by buying in the US.
I had the option of purchasing my iPhone 5's from either Canada or the US, but because the unlocked version isn't yet available in the US (and the fact that I'm impatient!) I ended up ordering mine from Apple's Canadian store.