Assuming that both parks are open at the time, there will be a direct park-to-park bus, but the ones that go to the MK actually go to the TTC.
Your fastest bus route is probably to take the bus from AK to the Contemporary, and walk over to the MK from there. (The walk is about 4/10 of a mile.) The bus ride would probably take about 24 minutes including possible wait time.
Taxicab would definitely be faster, however. The cab would probably take about 15 minutes, and again, you would want to have it drop you at the Contemporary, as that is the closest that a cab can get to the MK gate. (The official drop-off point for the MK is on the Poly side of the TTC, which means that once dropped off, you have to take a monorail or ferry, which again can involve some wait time.)