It will depend on the vaporized flow in te separator, while vertical separators are more convinient for higher vaporization than horizontal, if your separator is handling from 60-85% liquid then you should use the horizontal if less then the vertical will be a lot more eficient. Efficency will also be determined by your flowrate, if it is high, then the vapor will carry the liquid to the top and your separation wont be as good as you wish.
Efficiency is a two edged sword. What purity outlet do you need? How much plot area do you have? What design pressure do you need? What is the range of liquid slugging and loadings? Equally important are distances between fluid inlet and demister, between demister and outlet, distances between fluid inlet and liquid surface, fluid vortexing (in verticals) & rolling (in horizontals), etc.
