Without getting into specific boards, I can share the main characteristic that separates my downwind board from my flat-water board. The downwind board is a newer SIC Bullet 14 which has a planing entry with a fair amount of rocker which is perfect for dropping into and surfing steep bumps off the coast of Maui. The flat-water board has a low and very narrow piercing bow with no rocker so the sharp vertical entry is at water level. Though the boards are the same width with very similar rails, the Bullet is slow and hard to push through the water due to the flat-bottom/rockered entry but the board comes alive and takes off like a rocket when surfing good-sized bumps. or small ground swells. The flat-water board feels frictionless on flat water and glides fast and beautifully with each stroke, pure joy on flat water. It is fun to down-breeze on lakes with small whitecaps but that low sharp bow would be a big problem in decent sized ocean bumps or surf. Though I love both boards, the very different nose entries mean that neither one could be considered a combo downwind and flat-water board. On these boards, the nose entry has much more effect on speed and efficiency in different conditions than overall width or other factors (related to your question, both boards have identical square tails).
I hope that gives your curiosity something to play with.
