Depends on your personal car and what you plan on renting.
If your own car is already paid for, you're loosing.
If you have a recent lease, you might gain because of extra $$ if you go over total mileage allowed for the term.
If you own a recent car, you're loosing because the fixed expenses don't go away on your own car while you rent.
In Montreal during that weekend, rental cars will be as rare as hotel rooms, and as pricey, meaning get your credit card ready! If you rent in your home town, the price will be better.