AC is your only bet if you want to fly direct. UA and US will require transfers in ORD, IAD or PHL, CLT respectively. In the case of US, most of the flights from YYZ to the connection cities are on RJs so the only chance of upgrading is from PHL or CLT onwards.

In the case of UA, there is a possibility of upgrading but you'll need to build up a few miles before they kick in. If you can get a status match, you might score a few coupons, but unless you are going to fly a lot, you might put your AP status at risk. YYZ-MCO is around 1000 each way direct so unless you are doing a lot of other flying, spreading your miles over two programs might leave you in a lurch.

I was doing MCO on a regular basis last year and while it has been a few months since I've been down, I have never technically had to fly in the back...touch wood...Usually I upgraded with certs, but I've been comp'd more than a few times. The technically in the back comment refers to a couple of trips where i was on an all economy 320 that had no front and back .

Overall I'd say the choice is really between going direct or flying the connections with the hope of an upgrade. YYZ-MCO isn't a very long flight and unless you are in a middle seat in the back, it should be bearable on anybody's metal. Keeo in mind this comes from somebody who regularly endures YYZ-DFW in and RJ

DF