Not Or. And.

Fries with a mustard mayo mix, or a mayo ketchup mix, or a ketchup mustard mix, or a mix of all 3, are all fantastic.

When I do burgers at home, I do a mustard mayo mix on the bottom bun (followed by lettuce and tomato slices), and a ketchup on the top bun.

All 3 three bring something to the table. It the variety and proportions that give you so much flexibility in creating amazing food.