You definately will need the car to get to Deruta. We went there once by bus from Perugia and spent the WHOLE day there - and there isn't much there - as bus service is limited. The main drag below the old town is where the ceramic stores are. You can see them all to compare styles and prices. There are several stores up in the old town, too, as well as a ceramics museum.
It will be more expensive to rent for one day, but it is possible to drive from Rome (traffic is really not THAT bad...) Have you thought about spending two nights in Umbria and seeing what is there? Torgiano has a nice wine history museum, Perugia has several museums and churches, Assisi is a great hill town, and there are lots of neat, small towns to see. There are reasonable accomodations outside of Assisi, among other towns, and you could see a different side of Italy.
While Deruta does have a large supply of ceramics, Orvieto does, too. It is easier to get to my train.
It's hard to fit everything into an itinerary sometimes...