It's pretty hard to swallow a blanket statement without all the details:
Paris to Aix:
Car: You're off on the estimated times. Paris to Aix is seven hours of road time. Stops are extra. Gas will run about a hundred bucks and tolls around seventy-five.
Train: Three hours. One hundred fifty to two hundred fifty dollars for two tickets depending on departure time. In either case, you'd have to add on twenty bucks for the daily car rental price since you'd be using two modes of transportation on the same day.
Analysis: The train is either a wash or more expensive in cost. The train in better for time as long as you don't want to see anything along the way - - and there's plenty of that. My car costs are real close to dead-on - - assuming a forty mpg small car. Anybody could refute my train costs in either direction. If you get better train numbers, plug them into the matrix in lieu of mine.
Aix to Chartres:
Car: The same seven hours with the same provisions. Tolls will go up by about ten bucks. Gas will be about the same. Call it one eighty-five.
Train: Six hours with at least a Lyon to Montparnasse change in Paris and possibly one in Valence depending on departure time. Cost would range from one seventy-five to two seventy-five, depending on departure time. You may want to stick in the day's care rental again for getting around - - your call.
Analysis: Same as above.
Car Rentals.
AE is not necessarily the best source since they're a consolidator and have no cars - - you have to live with who they foist you off upon. They require you to pay up front, so you have no leverage to shift to the next counter and get those guys to match the deal - - they always will.
EbC has the same drawbacks with an additional problem - - you can't get an immediate quote and have to go back and forth by email.
There is no need to reserve far ahead. Prices will be about the same for a distant future date as they will be for the same date on the day previous, or even on the same date. What you want to do is reserve on-line instead of walking up blind to the counter. I take care of mine at the departure gate when there's nothing better to do. If I screw up, I do it on the landing roll. If I really screw up, I do it a couple feet from the counter and go snag a cup of coffee to give it time to enter the system - - sometimes I have to wait at the counter in that case until the computer coughs. The price is always about what I'd expect.
You'll have a larger selection at Avignon than Aix, especially if you'll need an automatic. If you switch, neither the times nor the costs in the matrix will shift appreciably.
Unsolicited Observation:
Aix is a royal son-of-bear for driving in and out of, especially if you're staying inside the one-way ring around the historic center. Nimes is best. Avignon in the middle, but closer to Nimes in ease of workability. As an example, I can get from Nimes to Apt in the same amount of time as I can from Aix, although it's almost twice the distance. You'll have your own reasons for selecting Aix, but it's a thought. On the other hand, if I had to rank the three cities for likeability, it'd be Aix, Nimes, Avignon from top to bottom.
