My preference would probably be to take the flight to Sarnia, if the price is similar to renting a car. I'd rather not step off a trans-Atlantic flight and drive for three hours in a foreign country, and then have to drive three hours on the way back too.

Customs in Toronto will not be more than an hour -- usually it is relatively quick and certainly less than an hour, but just how long depends on what the passenger load in the airport is at the time of your arrival.