Wait for the Sarnia flight, full stop.

To pick up a rental car you will need to shuttle to the rental location and wait in line to pick up a vehicle with all the controls on the wrongs side, then drive through rush hours on the right side of the road for 250-300kms in stop and go traffic until at least Waterloo or London when your body clock is at late night.

If you fly the 1840h flight to Sarnia you will be resting in a bed before you arrive by car assuming everything is running smoothly.