Antenna selection is a little topic all on its own.
Primarily though, the main differentiators are going to be the horizontal (azimuth) and vertical (elevation) beamwidths, and gain.
You would select an antenna based on your 'needs'.
So given that, I can't directly answer your first question because I don't know what the 'needs' are.
As far as placement, when using any sort of antenna that has directionality (a non-omni antenna) you would face the antenna as directly as possible facing the area you want covered. It's directional....so you 'point' it in the direction you need.