In short, I've found that when you mount road levers horizontally, it's better to use bigger, bulkier aero levers because they provide a larger surface area to grip or rest your palms on, tend to have better hoods for the purpose, and more intricate shapes that end up giving more grip opportunies. Traditional levers are less good because they typically have smaller bodies, and when viewed from what is normally the side there's more exposed lever arm, and less available body, than aero levers.