I've eaten at both multiple times and would choose Monsieur Paul. The food and service is superior and the atmosphere is really lovely. MP is a special experience. When you walk in, you are escorted upstairs to a beautiful, quiet dining room overlooking the France area of WS. Fresh, homemade bread, great food, super experienced wait staff. Le Cellier is good, but it's louder, more chaotic and the tables are very close together.