To me, Biergarten and Les Chefs is like compairing apples and oranges. Different food, different atmosphere, one is a buffet the other is not. So to me they both have their pros and cons.
My girlfriend is not fond of French food and would choose Biergarten over Les Chefs. The atmosphere is a lot of fun and the entertainment is pretty good. I think it matters who you sit with or the type of people you are. On our last visit we sat with a mid-aged couple and another family. The family sat there and didn't say a word. Not to us, not to eachother. It made things kind of awkward. The mid-aged couple were very friendly and a lot of fun. The 4 of us had a great time eating and drinking together.
As far as the food goes, Les Chefs has alright French food and Biergarten has alright German food. IMO, neither of them are better than the other.