It depends on the carrier you are flying. I don't know about all of them, but Air Canada, United and Canada 3000 all fly direct YYZ to MCO.
My guess is that they will book you on whatever is the least expensive, (currently my guess is Delta and Continental) and you will likely have a stop at one of their hubs, most likely EWR, CIN, or ATL. One warning, they might not get you advance seat assignments, so get to the airport EARLY! Like at least 2 hours early!
Anne