AFAIK, the Q400s for both WestJet and Air Canada are the exact same propeller aircraft.
You are probably thinking of the Dash 8s which yes are a little long in the tooth. From an economics point of view, I am sure they are worth their weight in gold, though.
Also, Aeroplan would never market these as business class. You can get J points from YLW to YYZ in J but your first leg will be in Y (it will be noted on Aeroplan's website that this portion would be in Economy). When you book a ticket, the points are not calculated segment by segment but rather origin to destination. So if you get a J reward but one leg doesn't have J, you do not get a discount for that.
As folks would say, speak with your wallet. If you prefer the 737s out of YLW go for it. It is only an hour flight, which I am sure connects you to some more modern aircraft .