Mono and Poly is a type of silicone the solar panels are created with from a crystal then compressed. Monocrystalline cells are cut from a silicon ingot grown from a single large crystal of silicon whilst polycrystalline cells are cut from an ingot made up of many smaller crystals.
When comparing panels of the same rated power output, there is currently no practicable difference in the performance of these two technologies. The only difference worth knowing is that if you are dead set on buying the highest efficiency panel on the market, the most expensive, highest wattage panels are monocrystalline. But the only advantage of a more efficient solar panel is that you can get more watts of PV on your roof. If you can fit the system size you need on your roof with more affordable panels, then there is no difference between mono and poly.