It is interesting that this is about what goes on it because that is the difference to me with Burger King and Whataburger. BK has the patty down and is far superior to Whataburger's patty but they lose it with the toppings. BK is mayo, ketchup, lettuce, tomato, onion slices and pickles. Whataburger is mustard, chopped onions, shredded lettuce, tomato and pickles. Dicing those onions and shredding that lettuce is a huge difference maker as is the mustard.
It also matters as to which one you can best eat while driving, Whataburger is better put together for a one handed dining experience.