Biergarten has atmosphere, and the kind of atmosphere that can be very unique to WDW. Le Cellier's atmosphere is more urbane, but nothing that I am impressed by on that point alone.
Food-wise, Biergarten is weaker, but I like that it's different. I can't always get to a German restaurant, especially one that is themed so thoroughly. LeCellier has good food, though like someone else said, I wasn't in love with it; I don't see too much of a difference in dining experience, compared to what I can get at home at a nice steakhouse. But it completely depends on what you're looking for.