actually the selection of polarization depends upon the different prameters.
Vertical polarization is more effective against rain.
Vertically polarized waves are less effective to multipath fade in lower frequencies.
vertical polarized wave give us 30% more RSL as compared to the Horizontal polirized wave.
The reason for using both H & V polarizations is to get maximum use of frequency in th network with out interference.
Hopefully this will help you