I use mono unless I am nymphing or I am fishing to very spooky trout. The reason is that if I break off mono, it will disintegrate over the years due to UV and weathering. Fluoro does not and will be around for literally hundreds of years. Plus mono is much cheaper.

Fluoro is an advantage when fishing to spooky fish with dries. A floating mono tippet has a different index of refraction than the water. It also depresses the meniscus slightly. This cause specular reflection and refraction. These bright spots can be seen as the fish look up at the floating mono and they can also be seen on the bottom of the stream in slow moving waters. With sunken fluoro tippet this does not occur.