PV modules are divided into two groups: monocrystalline and polycrystalline. These used to have considerable differences in the past, but the gap between them is shrinking as the technology advances.
Monocrystalline panels, or simply mono, where used to be considerable more expensive, with greater efficiency. Today the price difference between mono and poly panels is only noticeable in small scale projects.
In terms of efficiency and performance, although mono panels are said to be the better of the two, but averaging the system output over 20 years yields almost same result for both types.
Aesthetically, mono panels are the proffered choice. Poly panels as the name may suggest, are not uniform in color, whereas you get a nicely distributed color in mono panels.
Mono PV Module (above); Poly PV Module (below)
Therefore the three factors to consider when choosing between mono and poly are, in order of importance, price, aesthetics, and performance.