Dual seat for me, looks more finished and the chrome grab-rail looks great. The gap between the rear of the single seat and the rack also looks a bit jarring.
Also as C_S says with the single seat it looks like an old police bike painted green.
All imho, ymmv, etc.
As a flat-tracker that looks bad, too much engine and cooling kit takes away from the svelte lines of the tank and seat. Not sure the mono shock suits it either.
That merc is lovely. Buy it!