2 hourflight but you have to get to the airport early. Then you have to go from charles de gaulle to the city center. It's really much more than 2 hours. Also, in my opinion, it's a lot more stress. I'd prefer to hop on a train from city-center Rome and get dropped off in city-center Paris. Way easier. Also, trains have bar cars. Those are fun. Midnight Trains: sign me up!
For long distances, a flight is quicker. But for most of Europe, the distances aren't far enough to warrant a flight (over you add in the pre arrival time and waiting for checked luggage, and as someone else pointed out getting to and from the airports.)
I did that one year, traveled all over Europe by plane. By train is much more comfortable. And for the most part not much slower