Only 3 days after Bayeux... Mont Saint Michel or Loire Valley?

You can't go wrong with either! If you do decide on Mont Saint Michel I would recommend spending the night on the Mont if possible. You might also check the tide times and see if they are favorable for seeing the tide come in.

Both are wonderful but very different. If u like castles and countryside pick Loire.
But if u do not like castle hopping pick msm. Realize that in msm, a gd portion is touristy with souvenir shops. Both rewards come with venturing farther up to church, cloisters and meandering.

there are chateau everywhere in France including several in or around Paris; there is only one sight like Mont St. Michel. I'd go there and pick up St. Malo as well if you have time.
https://janettravels.wordpress.com/2014/01/21/mont-st-michel-is-that-a-real-place/
the chateaux of the Loire are magnificent so obviously go with your heart here, but for me it would be MSM.

If you go to MSM, perhaps you would also have time to visit some of the towns to the north and sample some of the seafood. Peter

We did both last June after seeing the WW2 beaches and staying in Bayeux as well. We stayed on the Mont.....a bucket list feat for us but the tide did not come in that night. Even so it was a night we will always treasure. It does not take long to do MSM.....we drove in late one afternoon as the tour buses were leaving and left after breakfast the next morning. From there we drove to Amboise in the Loire Valley. I must say that we ADORED the Loire Valley. We stayed in Amboise at Le Manior les Minimes. It was incredible....like staying in a friend's comfortable home. We had planned to stay 2 nights and do the castles but the Seine flood closed one and we stayed a third night there to avoid road delays and detours on our way back to Paris to fly home. I cannot say enough good things about Amboise.....perfect base to see all of the castles and lovely countryside. I would suggest that if at all possible you make certain you see Villandry.......the gardens are something you will never forget....and we loved Chenonceaux. It was our favorite castle. Have a great trip!

MSM is, usually, an overnight experience, so the suggestion to combine with a dip into Britanny is well taken. Or you could make a quick stop in the Loire after MSM.

To my mind the question probably hinges on how you really feel about (mostly empty) Renaisance chateaux. It's a personal decision, in other words.

If you choose Mont Saint Michel, I would also recommend to extend the trip at least to a part of the north coast of Bretagne (Brittany) : Saint Malo, Dinan, Cancale, Fougères, Rennes... (the Pink granite coast may be too far, or you keep it for a next trip dedicated to Brittany!)

Ideally a trip to north west of France would be 3 days in Normandy, then 3 in Brittany, and 3 in the Loire Valley (even if in every region, you could afford more than a week!).