It depends....
Bus if you do not like traffic, are easily lost on WDW property, do not want to deal with keys. In general, buses are the way to go. They move lots of folks and 'green' the race. They pick you up (I think SSR is one location but others can weigh in if I am incorrect) and drop you up front at the expo. On race day, you are dropped in the Epcot parking lot as well as picked up in the lot. Bus if you want to experience the first sign of post marathon legs - that is the real funny feeling of walking down the steps for the first time post race.
Drive if you are a little more of a control freak or if you are standing for a long period of time with no buses. If the pre-race weather is not so good and your car can serve as a weather hut. Only if you understand how to negotiate your way to Epcot in the dark while not following the signs.
The truth of the matter - for both forms of transportation you need to leave your resort between 3 and 4. Do not think you can sleep in and drive up at the last minute. From the SSR, you need to exit the guard shack on the DTD Buena Vista side of the resort. From there circle around DTD and head towards the Boardwalk resort. Pass the DHS and BW as well as the light by the Swan/Dolphin and backstage DHS. Be in the right lane and exit onto World. Once onWorld, stay in the right two lanes. Traffic may feel slow here but keep right as it will move and you will soon be in Epcot Parking Lot. Once there follow the parking attendants to park. Once parked follow the lights.
Note that about every other to every third year, leaving the parking lot is a mess. Not sure if there is an exact cause, but I have seen the parking lot backed up tight.
I have driven about as much as I have ridden the bus or monorail. Since this is the year where we should have relatively warm temps, I would think the bus is in order.
Though really I am saying it is a personal preference