Maybe your Ontario based insurance company may complain (deny coverage) if you claim a loss in the US if it’s discovered the car was principally garaged in the US. Your premiums are based on ON rates, which, for a Canadian insurer will always be lower than anywhere in the US – even if a US insurer will insure for less. Go figure.
You could always tell your insurer that the car will be principally operated in the US and pay the extra premiums – but expect to pay through your a$$. Maybe a US insurer may agree to insure the car with ON plates. Folks at customs will ask questions. If you are maintaining a principal address in Ontario, you should be fine. After all, ON address = ON drivers license.
Without more information, its impossible to answer your question. I leased in ON and kept my car in NJ for the full three years. My insurance was very expensive. I serviced the car whenever I went home to visit family, etc. Under NJ law, and similarly NY, you are supposed to register your car here within a couple weeks of moving into the state. That will be impossible for you to do if you lease/finance.