In everyday technical terms, the only slight difference is the efficiency of two types, for instance we have both types of Solar Panels in 60 cells assembly per module with exactly the same physical size (i.e., 1650mm x 992mm), in Mono series the highest model is of 270Wp while the highest model in Poly series is 260Wp, however if we take a 250Wp model of any type – there is no technical or practical difference.
The difference will only matter if you deploy highest power model in a mega scale project where thousands of Solar Panels are installed, say if you have to build a 10MWp plant using our highest model from two types, you will need 37037 numbers of Mono and 38462 numbers of Poly type. In this case the difference is significant – 1425 more panels are required in case of Poly type, this will translate in to increased area for installation, that much extra mounting, cabling etc., hence eventually the cost of project even using the less expensive type (Poly) may result in more overall cost.
There is another but almost insignificant difference in two types that is of temperature coefficient which goes in favour of Poly type.