OEM Aerodynamic options (to 'S' or not to 'S')

[invol]

Hello all! This is going to be an obligatory awkward first post.

I'm going to get a Yaris soon (if I can ever sell my current car), and was wondering which options would be best to get from an aerodynamic / fuel efficiency standpoint? I'm definitely getting a 3dr LB 5-speed (non-negotiable ), and I've been leaning towards getting an S mainly because I want the tach and the better seats. However, is the S any more aerodynamic than the base? Plus will the extra weight of the skirts offset any benefits? I'm not planning on getting any 'power' stuff (windows, alarm, etc.) save for the stereo to keep down weight a little. Are there any options that would help in my aero / FE quest?