Persian not something else 
Using the word Farsi which is the local name of the Persian language is like saying: "I speak Deutsch and can write Polski" instead of: I speak German and can write Polish.
And, to quote the distinguished Iranist Kamran Talattof: "The word Persian in the mind of an English speaker, consciously or not, recalls many other historical and cultural legacies about Iran. Persian is closely associated with Persian poetry, Persian carpets, Persian cats, Persian poetry, Persian pistachios, and so on. When you refer to this language as Persian, the audience may associate it with one or more of these relevant ideas. On the contrary, the word Farsi not only voids these historical and cultural associations, but it also adds to the recent portrayal of Iran as a strange and distant society."