A resort bus will take you to the front gates of the MK, not to the TTC. I would take the bus if I were you. If you take a taxi, it will have to drop you off at the TTC. We arrived by car at the TTC at 7:10 and had no problems getting on a monorail.
Just as a side note, we had an 8:10 ADR at CP and was questioned by a CM at the gate of MK. He didn't check our names off a list or anything...he just took us at our word that we had dining ressies.
Have a wonderful trip!