With two monitors your not going to be gaming across them anyway (imagine the monitor bezels running though your crosshair), so really it boils down to the screen real-estate provided, convenience of the setup and price.
In terms of sheer pixels the dual 1080p setup wins, for gaming I would say the 1440p wins because its just simply a higher usable res, convenience the 1440p as its a single screen, price the 1080p setup since a 1440p at its cheapest is typically ~$650 (Unless you go for a Korean model).