I'm not sure how the Canadian phone companies work - but if your phone is unlocked, you could purchase a sim card from a US company, put it in your phone and you're all set.
Then you've got unlimited talk, text and data.
Personally I'd recommend Visible - which is what my wife and son have. It runs on the Verizon network - which has good coverage. ATT (or one of the resellers of ATT) also has good coverage.
Straightalk is available through walmart I believe.
Once the sim is in your phone, you're all set.
Last - on some of the newer iphones, you can put in a 'virtual sim' - which I don't know how to do that first hand, but it supposed to allow you to have two phone numbers in one iphone.
I'm sure someone smarter than me will provide you with info on how to do that so you get the best of both worlds.