I have done both, and more than once each.
CT is basically a sampling of just about everything being served that evening. The items that are extra cost in the main dining room (caviar, foie gras, Kobe beef) are included. Dining at CT takes 4-4½ hours compared to the normal 2½ hours in the main dining room.
The downside is that CT, as it is in an alcove of the kitchen, is noisy and brightly lit. The main dining room is subdued in both lighting and sound level, with entertainment provided by a harpist who takes requests. (Have you ever heard the Mickey Mouse Club March played on a harp?)
If you are a serious cook, as Judy said after our first CT visit, it will be the best cooking lesson you will ever have.
The main dining room generally has 30 to 35 reservations per evening (15 to 18 per seating) while there is only one reservation per evening for CT.
For romantic and quiet the choice is clearly the main dining room. For an experience unlike any you have ever had go with CT.