I have found 2 companies which run day trips from Paris to Normandy, Viator and City Wonders. Is one better than the other?
Where in Normandy do you want to go? And why not just take the train?
City Wonders. Victor is an aggregator and does not actually run tours. Odds are your tour will actually be with City Wonders but there will be a third party (Viator) between you and City Wonder. Better to book directly with a service provider when possible. I have close friends whose judgment I trust who used City Wonders for a day trip to Mont Saint Michel (14 hours) and they gave City Wonders high praise. But for a different view on City Wonder see word-was0z’s post at https://community.ricksteves.com/travel-forum/france/does-anyone-have-experience-with-city-wonders-tours-from-paris. Either way, I think Cty Wonders is the better choice from your two options.
What is it in Normandy you want to see? If it is Honfleur or Rouen, you could easily take a train and visit one of them in a day. If you want to see Mont-Saint-Michel, doing a day trip will ensure that you are nearly crushed to death by the mid-day crowds. If your interest is in WWII history and the D-day landings, a day trip is not nearly enough time, and you will spend the bulk of the day simply getting there and back.