We will be traveling from Rome to detonate to spend a week in Tuscany..is it better to rent a car on maybe the out skirts of Rome and return it to the Florence airport or is it better to tajke the train to someplace like Orvieto and get a rental car there?
If you decide to pick up a car, definitely look to pick-up on the outskirts of Rome. You will dave yourself quite a bit of hassel...and time!
However the choice between train and car is really personal - if you like the idea of having a bit more control of your timeframe then you will probably enjoy having a car from the get-go. But having said that, the scenery and places to stop along the way are not very enticing.
If you do decide to pick-up a car then deciding on Rome or Orvieto probably come down to checking out the costs. Orvieto has car rental options - but remember to calculate the train tickets you will by, transport to the rental agency and the time it takes you to do all of this. AND, be sure the rental agency is open on the arrival day (double check the hours)
Orvieto is definitely a place to visit - I have been several times and love the sites, the food, the wine and the atmosphere. You may want to consider staying a night here and then driving into Tuscany following the back roads (via Cassia) - yes, it will take a bit more time, but the sites and scenery are wonderful. Look at Bagni San Filippo and Abbadia San Salvatore for ideas of where to stop along the way.
If you took another night to spend in Chianti, San Gimignano or Siena - you would get a great overview of the region and then you could leave your car at the airport. It would be easy to get to the Florence city center from the car drop off point and then if you have other day trips planned, you can rely on public transport to visit places like Pisa or Lucca.