The only noticeable difference is that a kilowatt of polycrystalline panels takes up more space than a kilowatt of monocrystalline panels. Similarly, monocrystalline panels are available as single panels at higher wattages, i.e. poly panels in the 330-350 watt range and mono panels in the 400-450 watt range. As a result of using monocrystalline panels, more capacity plants can be built in areas where space is limited. There is no point in spending a lot of money on mono panels if you have enough rooftop space. Don't fall for marketing gimmicks like "more output even in low light," "higher efficiency," and "long life."