Sloped downcomer designs do offer a greater active area (slight) but the main reason for spending more money on a sloped downcomer in order to direct liquid flow from an upper to a lower tray is when there is an issue with vapour-liquid disengagement - if there is a lot of foaming for example. With ethanol-water distillation systems this isn't a huge problem.
I would be more concerned with the downcomer sizing rather than whether its straight or sloped. It's more important to avoid downcomer choking and backup flooding problems than to get a small benefit from an increase in active area. (need to estimate the optimum downcomer area, average velocity and pressure drop - which are almost always empirically determined. Sloped downcomer make it even more difficult to estimate)
Nerd for the day :-B