Mustard is supreme to mayo and all others for so many reasons to include shelf life, flavor, smell, variety, texture, ingredients. It's something else and can be used in meat rubs and melted into cheese dips, so much
But mayo is supreme when it comes to a base for a mixture. You can mix almost anything into mayo. If I had to choose just mayo or just mustard for a sandwich, I'd choose mayo. But both of those together? Easily better than either by themselves.