aerodynamic, Pedals with toe clips and cleated shoes are equal to a clipless system when it comes to efficiency. I feel the advantages of the clipless pedals are that there is no reaching down to tighten or loosen the strap and there's usually a bit of float (lateral movement) when engaged. Bottom line, either system is more efficient than a platform pedal if used as intended. I've no clue as to what extent, however.

I use clipless systems on all of my bikes. My touring and mountain bikes have double sided SPD pedals and I wear mountain bike style shoes that have the cleat recessed for easier walking.

Brad