OK, to answer your question relative to two sails with the same total sail area, the one with the longer luff (and thereby a shorter foot) will be the one with a higher aspect ratio.

The new (to some) Square Top type mainsails you see on most newer designs (F-16, F18, A cats, etc.) were developed because with a fixed rule to total sail area, the sail makers figured out that they can generate the most lift by having a shorter foot and longer luff. In effect they were moving the sail area from the foot up to the top, in the square head, vs. the older triangle sail designs like you would see on most boats prior to the advent of the Square Tops.

So, if you have a choice but are limited to a rule for "Total Sail Area" you would be wise to have a "high aspect ratio" type sail made. It will have a square top and look much more like a rectangle than the older triangle type sails. It will also generate more lift and less drag.

You can also keep the same luff length, same mast, and just add the square top, shorten the foot to keep the same sail area.