Q:how cross polarised antenna is better than space div. antenna

Ans: 

Polarisation diversity Vs Space Diversity

It also depends upon the kind of clutter or area where you want to use these. Cross Polar antenna which has its dipoles placed at +45 and -45 degrees provide us better diversity in a multipath environment where we have lot of reflections,diffractions as chances are mostly that polarisation of the signal transmitted would change by the time it reaches Rx antenna and +-45 degree configuration of dipoles provide maximum probability that the signal will be matched to these phase difference. Another point in favour of crosspolar antennae in city is slant gain which is around 1.5dB. This comes from the angle at which we hold our mobile handsets (mostly at around 45 degrees). 
Space diversity first of all needs space between antennae atlest 10times the wavelength. Secondly the tower structure has to be strong and two different units have to be used. These are the physical limitations. But, it is useful for open areas or highways where chances are most of the times signal transmitted with be in LOS and without phase change.