For what it's worth, I live in Quebec City, and we flew QC-MTL-Orlando when we went to WDW in 2016, and will do te same for our upcoming trip in May. Flying economy out of QC, options are limited, prices will be high, and you won't be comfortable anyway you look at it
I don't have any preference of one over the other, for us, it was all about the time and prices and the flight times we chose with AC were better for us than what we could get with WestJet, and for a better price (at the time we booked).
I'm not very familiar with WestJet airplanes, but Air Canada Jazz to Montreal will be a small propeller plane, 2-2 configuration with maybe 15-20 rows. It won't be very comfortable, but it's a 45-minute flight so it doesn't matter much. Air Canada Rouge will be a "nicer" newer plane, but with small seats, not much legroom, and you need to install the AC app on your device for in-flight entertainment. It's not fancy, but, like I said, not that many options coming out of Quebec City, and whatever gets us to WDW is fine with me
Someone mentioned the train to go to MTL. If you're looking at cutting costs, you can also take the bus (
https://www.orleansexpress.com/en/) I don't think flight directly out of Montreal is significantly cheaper than starting in QC with a connection, but if you don't want to do 2 flights, buses are cheap, clean and economical. I think they also have a bus running to the airport.