Canadian looking to start a US-based business
In this case your incorporation decisions should correspond with your actual plans. Let me describe two scenarios as I see them:
1. If you see that it would make more economical sense to manufacture your product in Canada then you would have to register your company in Canada. That same company could be used to market your product in Canada, while for marketing of the product in the US you could then register a new US company. That US entity can either be owned by the Canadian company or by you personally, and would buy the product from the Canadian company and resell it in the US markets.
2. If it makes more sense to manufacture the product in the US then you would need to register your company in the US. If and when your product would be ready to be sold in other markets such as Canada you will have the choice to either register a Canadian company or a branch of your US company (I am not sure how this part works in Canada though, you would need to consult a Canadian tax/law specialist).