I plan on taking a day trip from Rome to either Orvieto or Montepulciano - which would people reccommend? I will be renting a car. Also open to other suggestions but am leaning toward one of these two
For Orvieto I would just take the train from Rome. Montepulciano would be a bit more complicated and take a little longer to get to. We enjoyed both towns a lot, we preferred Montepulciano, but we were not visiting there from Rome. Orvieto is what I would recommend because its easier and quicker to get to from Rome for a day trip. If spending a few days, then I'd switch that recommendation, but that would be a personal preference, others would still prefer Orvieto.
I have to agree with Nadine. From Rome I would take the train to Orvieto. Were you only planning to rent the car for that purpose, or will you have one anyway? If you will have one anyway, that might change my answer.
Orvieto is so much easier to get to with a quick train ride and has a wonderful ambiance. It's a great size, has a beautiful Cathedral and views, a farmers market and nice evenings with everyone strolling and Orvieto wine much better than we get here. Montepulciano felt more touristy with wine shops that got repetitious to me. Picking up a rental car, then driving and parking is a hassle in Italy. The wineries around Montalcino are great but even more work to get to. You may want to consider spending the night in Orvieto to experience it in the evening plus the hotels are much better values than Rome. The Rome hotel may be willing to store most of your luggage and you can bring an overnight bag only.
Liked Montepulciano, LOVED Orvieto. We, too, had a car and parked in the lot right at the tourist office where you buy the museum card and the funicular tickets, just below the St. Patrick's (Pozzo di San Patrizio) well if I remember correctly. We followed Rick Steve's sightseeing recommendations and spent a full, lovely day. There is plenty to keep you occupied in the compact are around the duomo. We loved Museo Faina, the Museo Archeologico and the views from Torre del Moro! We ran out of time and missed the "Orvieto Underground" tour. If for any reason you choose Montepulciano and are underwhelmed, visit tiny Pienza, just a few short miles away (and covered in Steves' book). A very good tour of a palazzo and incredible views. Enjoy!
Personally I'd opt for Orvieto. I agree with Maria; I liked Montepulciano but loved Orvieto. There's a lot to do in Orvieto & I suggest the hour long underground tour. The facade of the duomo is the prettiest I think I've seen in Italy, especially when the sun hits it & makes all the gold sparkle! I'd also suggest climbing to the top of the Torre del Moro; the views are amazing. We loved getting off the touristy path & exploring all the little back streets; it's a quaint and pretty little town. It's an easy 90-minute train ride from Rome. If you are renting the car just for this day trip, I'd skip the car as getting there by train is easy and efficient. I also agree with Maria about Pienza. If you choose to visit Montepulciano then perhaps you can work in a visit to Pienza for a few hours. I actually preferred Pienza to Montepulciano and I fell madly in love with the Pecorino Fresco cheese there!