Aside from aesthetic differences, challah doesn’t contain dairy products. Challah is also typically leaner, much leaner than brioche. The leanest brioche, aka poor man’s brioche, is typically around 20% butter (in baker’s percentages), while fats in challah amount to no more than 15%, usually. Challah is typically hydrated with a large amount of water and a small amount of egg, while brioche is hydrated with a small amount of milk and a large amount of egg. Challah usually contains an additional 2-3% of sugar compared to brioche. Challah is typically washed with egg only, while brioche uses a milk/egg wash.