It is definitely easier to get to both Sienna and Volterra from Lucca by car.  There is quite a bit of parking outside of Volterra so that is easy and it is a beautiful drive.  Parking and driving in Sienna can be a challenge, you just need to be patient.  There are huge parking lots outside the walls of Sienna so you would think it would be easy, but there are a lot of tours groups that visit Sienna so both parking and driving can be difficult.  But once you are there you can walk everwhere, you just need to be sure to remember where you parked your car.  If you decide to go to Florence from Lucca, it will be easier to take the train.