Efficiency
Because they use higher-quality, single-crystal silicon (see above), mono panels are better at turning solar energy into electricity. No solar panel is ever 100% efficient, but mono panels generally demonstrate the levels of efficiency expected from the latest technology (which, at present, are 15%–20%).
Power and space
If you have a limited amount of roof space, you need a system that will produce as much electricity for your home as possible. Mono panels tend to be more powerful, meaning you can install fewer of them and still achieve an output equal to a larger array of poly panels.
Lifespan
Most solar panel manufacturers give you a 25-year warranty, and this will apply whether you buy mono or poly. However, the silicon that goes into making the panels is durable enough that the panels are likely to last much longer.