Nowadays there is not much difference between mono and multi. Both of these panels are expected to last at least 25 years as they are both normally given 25 year performance warranties. Some installers will give you an option between the types, others might have chosen just one type after working out what will give their customer the best performance output for the cost.
Here's a quick run down between the three:
Multicrystalline solar panels, AKA polycrystalline have solar cells made of a number of small silicon crystals. As mentioned above, these types of panels normally produce less power per area size than monocrystalline panels, but the technology in these panels have caught up in power production and are now close behind monocrystalline panels. Given that multicrystalline panels are normally cheaper than monocrystalline panels, they often are the more economical choice.
Thin-film panels are made up of a thin layer of photovoltaic material deposited on a sheet of metal or glass. Because of the lightweight material thin-film panels are not expected to last as long, but are normally the cheapest type of panel.