You are going to get various opinions on driving/busses. But for the specific example of the CRT character breakfast, Disney transportation is going to get you closer/faster. If you drive your car to the MK parking lot before 8:00 am, you are not going to be able to park, take a monorail or ferry, and make it to CRT by 8:00 am. The Disney bus will take you directly to the bus stop by the entrance of MK. You can then walk over and give a CM your PS number for entrance to the park early.
You did not say which "Mickey breakfast." There are at least four breakfasts with Mickey. Which are you planning to attend? Is it at a park or a resort. If it is at one of the resorts, you may be better off driving. But if it is in a park, you would probably be better off taking Disney busses.