We usually use aires in France, no matter when we go. I hate planning.
I haven't driven to Spain for 25 years or so, however when we last went to France 2 years ago, I did price it up, we were heading towards Spain.
I worked out that diving both ways worked out about half the cost of the ferry, however we never got to Spain and only got halfway down France and spent 3 weeks there.
I would always drive but then I love France and didn't particularly enjoy Spain, but that's just me.
I have driven from Perpignan on the Med to Caen in one day before, so it can be done, but sitting with your feet up in a ships cabin is easier obviously.
I know Spinner is on the Portsmouth to Bilbao ferry now, so he will know more and I'm sure he will reply to this thread at some point.