If you can, consider flying from Buffalo.
I would not.
A friend of mine tried that and got stopped at the Boarder by some buffoon US Customs Agent on a power trip.
He had his passport, he had his ticket.
He didn't have his flight itinerary.
Boarder cop proceeded to play tough guy. Asked him where the Itinerary was. Friend said he had ticket in hand. Boarder cop says, "how do I know you won't stay in the country" Blah blah blah.
This is where it gets wierd - my friend says that the Itinerary is on his Blackberry. Boarder cop says, "if you show me that, I will have to keep it" I have no idea why.
Long story short - they missed their flight due to a moron.
Not worth 50 or 60 bucks savings frankly.
BTW, friend is middle class white guy with decent job, etc. Boarder cop was just being a prick like so many of them are. You're better to go through US Customs in Pearson.