The prime advantage of the loose foot is the ability to 'flatten'; not able to be done on a shelf foot without a flatttening reef added.

Within the design range of 'stretch' the shelf foot is more aerodynamically efficient as the shape along the foot is similar to the 'upturned' wing tip that retards vortex-shedding formation, lessens the 'shortcircuiting' of air flow 'across' the foot from the high pressure side to the low pressure side, etc. A loose foot sail constructed with an 'apron' (an extension of sail material that is added to the foot which 'hangs down') will almost do the same thing.

But the larger range of 'outhaul adjustability' (+ 'apron') of the loose foot thus better 'shaping' control .... the aerodynamic advantages of the shelf foot diminishes. and they both become equal. Since the loose foot can be easily 'flattened' (similar to old fashioned 'flattening reef' in a shelf foot - a PITA), the over all advantage has to go to the loose foot (with 'apron') simply due to the 'ease of use'.

That stated, most cruisers simply 'just raise' a (dacron) sail and hardly ever 'shape' a sail (for helm balance, power-up or power down, etc.) by other than outhaul tension.