We priced a transporter and it was around the same as the Espace. This rental is through Hertz, the transporter was through Avis.
Don't bother with the Zafira, it's no bigger than a mid size hatchback. Granted, it does have more headroom than a the Honda Civic, which will be very useful if you're all planning on wearing extra-tall top hats for the journey down. Other than that, the extra height was cunningly designed into the vehicle as a total waste of space and time. I'm guessing you won't be doing the top hat thing, and I'm guessing too that you are not dwarves either. So go for the Espace. It's more $ ok, but I reckon it's defo worth the extra. You may well be stuck in traffic in Paris, Rouen or LM itself for many an hour and a Z-car ain't the place to be for ten hours.
The Transporter isn't really a car, not in the true sense. It's based on a very sturdy VW commercial vehicle platform and has all the driving dynamics of a Ford Econoline panel van. Whilst worthy for hauling light freight, it makes for a very uninspiring passenger drive. If that's your bag, then fine. But so far as I can see, the Espace is the way to go. Big, comfey, quirky and surprisingly nimble it's the absolute epitome of what I think a French saloon car should be. Enjoy!