You may think that the choice between glossy or flat is only aesthetic, but there are other considerations too.
High Gloss: These paints usually contain more paint resin and less pigment which allows them to last longer and withstand weather better than flat and low-gloss paints.
Flat Paints: The risk on mold and mildew growth is higher with flat paints because they absorb water and collect dirt more easily.
When selecting a paint for exterior finishes, it may be worth your while to consider these factors.