I would think taking the bus from CBR directly to the Magic Kingdom would be quickest, since they let you off right outside of the gates.
If you take a bus to EPCOT and then get on the monorail, you'll have to get off at the Ticket and Transportation Center to get on the monorail to the Magic Kingdom.
So, taking a bus directly to the MK, you'll have to wait for 1 bus. Taking a bus to EPCOT then monorail to MK, you'll have to wait for 1 bus and 2 monorails. If you're not in a hurry and really want to ride the monorail, then by all means do it, but if you want to get to the MK the fastest, take the bus directly there.