I would vote for a US airline as well, just because it can be much more painful to clear customs in the middle of your flight, especially if your first flight is delayed. Plus, I really don't like Pearson airport in Toronto.
I have flown United, Delta, Northwest, US Airways. My favorite is Northwest because Minneapolis seems to be on the most direct route to Orlando of any of the airlines, and they have been on time most of the time. I'll probably fly NWA from Edmonton to US cities (including Orlando) this year about 8-10 times. Second vote went to US Airways that had entertainment on the Edmonton-Phoenix leg. This isn't always the case though.
My least favorite by far is United. They have lost my luggage more than all other airlines combined. The plane from Edmonton to Chicago (or Denver) is just too small for my comfort. Plus, more than half the time we have flown to Chicago, there is a weather delay. The problem is that United has most often been the cheapest alternative.
The only time we've ever used Westjet is when flying on airmiles. Otherwise, their regular rate is just too high. Our August trip would have been $900/pp with Westjet and is only $377 with United (ugh!)