That's a tough choice as we love both, but it probably depends on what you're looking for. Biergarten is more of a (and I hate to be obvious) Octoberfest kind of meal. A band plays regularily, people dance and you are seated with other people at long tables for the buffet meal. It is a blast and the food is very good. Some will probably disagree, but if you like German food, we think it is good.
LeCellier is more of a quiet, out of the way kind of restaurant (you really don't even know you're at WDW). But, it is our favorite restaurant in all WDW. The food is excellent and the CM's are great. Don't miss the pretzel bread sticks and the maple glaze filet mignon.
I guess I would say, we never miss LeCellier, but some trips we do miss the Biergarten.