I have used Burlington with no problems at all form Montreal. You can get flights from there to Orlando for about 200-250 CDN.
I would not think you would want to spend 3.0 (1.5 each way) days on a bus with a large group of people eating a varierty of possibly interesting smelling foods, sleeping and sweating and smelling like humans do after a day and a half, all sharing one bathroom.
You can get a bus to Burlington for about $50.00, so maybe 300.00 CDN total. but an extra 2-3 days in florida, and no food to buy for the 3 days of travelling, it has to be worth something.
You could even look into flying out of Burlington, and flying into Montreal.....
Also look at air transat.....from Montreal they can be super cheap sometimes.