I'm assuming you'll be having breakfast, right? Rather than dinner. So, I would take the bus. There are buses that start very early for those character meals...so if you have an ADR at 8:00, you'll want to be out at the bus stop by 6:45 or so. The buses don't run quite as often as they do during regular hours, so if you just miss one, you may have to wait up to 30 mins for the next one.
The nice thing is that when you get to MK, at that early hour, you'll find a pretty empty Main St and some nice photo ops.
If you choose to drive your car (and it will be quicker to drive) you may have an issue getting from the TTC to MK. I've heard many stories from people who were not allowed to get to MK from the TTC. The park doesn't open until 9:00, so they don't start the ferry and monorail until around 8:15ish. Even if it's an EMH morning, they don't start because they assume the resort guests will be taking the buses. So, don't take the chance. I would just take the bus and have a nice time on Main St. early without the hordes of people.