Over the years we have been to both and were disturbed mostly at how Mont St. Michel has changed. It has become more of a tourist trap than anything else and the chapel is hard to find for the public unless one is determined.
Chartres OTOH is still a center for pilgrimages and is fairly intact.
Sadly, the beautiful cathedrals, basilica throughout France at least remain so only in outward appearance, inside being more similar to older train stations. The beauty and majesty inside are usurped by more modern trappings, sometimes modernistic banners and feel-good stuff tacked around, the tabernacles gone, and even the big crucifixes in the back rooms.
Shells, for the most part. IOW.
Worst of all, we watched the gradual deterioration of Lourdes from a spectacle of solid faith decades ago to a kumbaya center for ALL New Age faiths. Haven't been back now for nearly ten years.
Encouraging, though, the core of solid faithful is there and defiant. Despite the best efforts of the anti-church elements, it prevails.
Sigh........ :-(
Europe seems on its way to worse. Here is the Hagia Sophia, Church of the Holy Wisdom and crown jewel of the Byzantine Empire in the City of Constantine. Older than Mohammed, it survived as a Christian edifice until A.D. 1453, a reign of a thousand years. Conquering Turks were at last able to take it, and they destroyed its ikons and converted it into a mosque. This despoliation is still obvious because of the four minarets. Today it is a museum, in a country (Turkey) something like 99% muslim, but secular.
The way things are going, Europe's many great cathedrals may well share this fate.
A dance call the Thousand-Hand Guanyin
Give it time -- slow to load.