St chappelle or Chartres

Terrence, if you have the time, the upper chapel is worth seeing. The extensive stained glass windows around the perimeter is most beautiful and impressive. St. Chappelle was built by the only French King to achieve sainthood. We even named a city after him. Saint Louis IX.

One is a chapel and the other a cathedral. One is in Paris and the other is not. If you don't mind the trip to Chartres, then its cathedral is worth a few hours of your time. Sainte-Chapelle, if you will be in Paris, can be visited and appreciated in an hour or so if you are short of time.

Chartres cathedral is about a 1 hour train ride from Paris. It is located close to the train station there and easy to walk to. The town itself is very charming and the cathedral was magnificent with unbelievable stained glass windows, all original. St. Chappelle is beautiful little chapel with great stained glass also, so if you aren't up for a day trip from Paris you will still see something magnificent.
Every visitor to Paris should see it at some point.

Chartres is much bigger and takes longer to visit (even apart from the hour or so train ride from Paris if that's your base). Malcolm Miller is an Englishman who gives a very good daily tour if your visit coincides with his schedule (Google him). Ste-Chappelle is in the heart of Paris, easily combined with Notre Dame and other nearby sights. But you may spend time waiting to go through a metal detector, since it's in the courtyard of a building that houses law enforcement agencies. Chartres will take more time, both are well worth the time they take, imo, but you have to set your own priorities.

You can go to an evening concert at St. Chapelle. Get there early and you can wander the upper chapel and take photos before the concert. (and you will avoid the long security line during the day) There are often 2 concerts in an evening, choose the one that puts you there at sunset, so you'll see the chapel in daylight. It's lovely to watch the stained glass windows as the sun sets.

Both are magnificent, but different. I've never seen color like St. Chappelle, though. If you're in Paris, this is a no brainer. If you're willing to train it to Chartre (not difficult), then see that one too.