With identical ingredients, would you be just as happy if you turned your sandwich upside down before you ate it?
To me, the construction of the sandwich is almost as important as the ingredients. For example, if something is pungent or aromatic (such as horseradish or mustard), I like that to be on the top and closer to my sinuses. I also like lettuce on the top, especially if it is crisp. Mayo goes on the bottom. If the bread is toasted, the crispiest piece should be on top.
Any preferences?