You decide of course, but consider the following: if your flight doesn't leave too early in the morning, it might be worth taking an hotel in the city anyway. Just imagine that you want to have a nice dinner, and walk around a bit in the evening (it's Paris after all). How will you feel about the train ride back to the airport late at night? (And leaving the city already by 8pm or so would be a shame, since you'll only have one night in Paris before the world ends... )