As I wrote last December, the basic difference between the two approaches is that feed in tariffs specify a guaranteed price for renewable energy and are usually long-term, fixed rates that are not pegged to the retail price of energy. RPSs use a target or quota for renewable that is legislated and determined by policy regulations, often based on a system of tradable renewable credits and bidding processes for companies, with the value of the credits determined by a wide range of factors. Mendonca describes FITs this way: "they place a legal obligation on utilities to purchase electricity from renewable energy installations. The tariff rate is guaranteed, and in the best examples, for a long period -- say 20 years. The tariff rate is scientifically determined for each technology, to ensure profitable operation of the installation."
I think the FITs argument makes sense, and shows how committed a government can be to providing energy to its people while doing as little damage to the environment as possible. I recommend reading Mendonca's article in full.