We've eaten at both - YSH twice and Le Cellier 3 times. That said, we enjoy YSH more. Not only is the dining room at YSH more spacious, but the food quality seems to be consistently good. We've rarely been at YSH when it was crowded (been both in June of 2007 and March of this year, the week following Easter Sunday). When we've been to Le Cellier (Christmas week 2006, June 2007, and October 2008), it's always packed. My risotto was undercooked 2 of the 3 times (I get the mushroom filet), the beer cheese soup tasted "off" in June 2007 (almost like the cheese was outdated?). The chocolate "moose" is always good, though!
You pay more at YSH, but I feel that the meal overall - the food quality, the presentation of the meal, the atmosphere and decor - put YSH at a notch above LC. To me, YSH is worth a 2 table service credit (we are DVC members and thus use the DDE, but still), whereas LC is not. Both are good, but LC is hit and miss for us, whereas YSH is always spot on.
A caveat: We are firm believers in not taking others' word for it bur rather experiencing a restaurant for yourself. This explains why we love Coral Reef and Teppan Edo while many DISers do not. We also like Boma, California Grill, Prime Time Cafe, the food at Hoop Dee Doo, Crystal Palace (dinner), Citricos, etc. We do not care for Hollywood Brown Derby, Kona has been hit-and-miss, Narcoossee's, etc. (all as a point of reference). In short, experience it for yourself, no matter what others on here may say. Wherever you go, enjoy!