As an alternative to a bus, one of the Buffalo limo services offers a town car service that will pick you up at Pearson and take you straight to Buffalo to a location of your choice. It costs $175 US (not including gratuity, tolls or fuel surcharge) one way, or $225 all inclusive. PM me if you'd like the company details, as I understand links to companies are discouraged. I'm in no way connected with the company, by the way, just have used them a lot!

I've taken the Niagara Air Bus service before, and the price is good, but I can't recommend anything else about it. The last thing you need when you've got a flight to catch in Toronto is the bus stopping at every hotel around Niagara Falls to let the senior citizens on