On one bicycle I have Crankbros Double Shots.

I don't like having to check my foot position on the pedal; sometimes I catch my heel on the crank arm.

When I have to hammer some, I feel more secure on the clip in side.

Road bikes always clipped in.

Beater was only flats.

I don't think there's any advantage to using the flats side except I don't have to put on my bicycle shoes.