Please give me some advice if you have a preference of one over the other. I've read that there isn't time to see both in one day. Also, any advice about how long to stay there and how to get there would be great! Thanks!
The D'Este was terrific and took several hours to see all of the fountains, listen to the concert, tour the ville. See RoninRome.com for a very detailed and I mean detailed description of how to get there. Bus out and train back. Great day trip.
We did both in a day, and didn't feel rushed. As to a preference, do you like beautiful gardens? Villa d'Este. Are you a fancier of ancient ruins? Hadrian's villa.