There are several differences between polycrystalline and monocrystalline solar panels.
Mono solar panels are made from a single silicon crystal, while poly is made from melting down crystals into multiple cells. This creates a visual difference in the panels. Mono panels typically are darker than poly panels, but a coating can be added to poly panels as well, giving them a similar appearance to the darker, mono panels. Mono panels also typically take up less space to produce the same amount of power than a poly panel – but again, this is not always the case. Mono panels gather more energy in low light, such as cloudy conditions, while poly panels can gather more scattered rays, such as in shaded conditions. Typically a mono panel costs slightly more than a poly panel due to the manufacturing process. They are also expected to last longer.
At the end of the day, choosing mono or poly panels should not be your main consideration when looking at panels, as the manufacturer of the product ultimately dictates quality.