It depends on how you do the parks. If you are staying until closing, driving may be your best bet. The busses are very busy especially if there is a big event at closing such as fireworks.
If you plan to get to the parks late, you could be parking far away and lose any advantage to driving while waiting on a tram/riding in to gate area.
While many give the busses a bad rap (It was well deserved in the past) it has gotten much better over the last 5 years or so.
One problem you may encounter at PO Riverside is that there are 4 stops in the resort and you could be on the bus 10-15 minutes before it ever leaves the resort. Add more time with wheelchairs and
ECV loadings.