There really is no comparison--LeCellier is head and shoulders above the Concourse, in terms of food quality, ambience, and particularly service.
The Concourse is OK at best, while LeCellier is one of the better restaurants in the Disney family. On our last two visits to Concourse the food was passable but the service was AWFUL. There is nothing special about the atmosphere at the Concourse--it essentially is a lobby restaurant at the Contemporary Resort, and can also be uncomfortably noisy as a result to its proximity to Chef Mickeys. IMO, its not worth a special trip to the Concourse.
On each visit to LeCellier, I expect the restaurants popularity to start effecting the quality, but, knock on wood, this has not happened. In my opinion, it actually has gotten better over the past few years.