It's really all a matter of personal preference. Both have thier pros and cons, it's just a matter of what type of person you are.
Give or take, both will take the same amount of time. (Exception: MK bus will be faster than driving)
With a car, you park at the TTC, and then take the tram to the Monorail, and then the Monorail to the MK. The bus takes you directly to the MK.
At night, the bus will have long lines. So will the Monorail, and the tram.
With the bus, you have to fold the stroller. You still have to fold the stroller with a car, and the parking tram.
The buses are air conditioned. The trams have natural air conditioning.

Your car has A/C, but will be hot for the first minute or two until the A/C starts to cool everything down.
Buses can get crowded during peak times, and sometimes it might be standing room only. You can always wait for the next bus (during peak season, it wont be long) Cars will be private, with a seat guaranteed.