Flipkart claims it's having the biggest revenue every diwali and Amazon never argues.It is not clear which portal makes more in terms of the total value of sales
One could speculate FK gets more lower-end sales whereas amazon seems to get the higher ticket items. In the main metros anyway.
For sellers it's not easy to sell to cheap, generic, copy items on flipkart like Amazon. For buyers too, the search on flipkart is too organic to find such items. Amazon search is highly inorganic and that helps in this matter.