I have some Kenzo Flower and a bottle of Royal Bain of indeterminate age except that it was still labeled Royal Bain de CHAMPAGNE. Yes, they are quite similar in the sweet, musky, billowing, vanilla creme soda body. Flower has more florals (tuberose?) and a more distinct animalic drydown. Overall, it seems to have more refined ingredients. RBdC feels more blended, in that I cannot pick out individual notes, especially the florals. I also get a hint of waxy paraffin in the start that's somewhat unpleasant. All of this could be due to the age of the bottle, who knows.
I find both to be quite warm and enveloping, like wrapping yourself in a thick bath towel. Flower is a bit more addictive and also more feminine. RBdC seems pretty unisex. Longevity and sillage are powerful in both, which is unusual for my fragrance-eating skin.