To the OP if you are traveling by yourself the Aerobus is fantastic.
If traveling with more than yourself then a taxi starts to make sense dollar wise.
I am a frequent visitor to Montreal and love the Aerobus,but on my last trip their was a group of three german tourist travelling together with a lot of luggage-they would have been much better of in a cab IMO since their hotel was not one of the scheduled stops so they had to offload their many pieces of luggage and wait for the transfer to thier hotel.

To the ft'er wanting to visit in winter.It is very very very cold.And ICY!!!Walking those cobblestone streets of old town is not a lot of fun IMO
But it is picture postcard beautiful and their are fantastic hotel rates in January.
DON"T even think about driving!I know the locals do when they must-but unless you are skilled in driving in such conditions(as a native Californian I know I am not)it is nothing to even think about.