On the one hand, much of Farsi is simpler than Arabic and, being an Indo-European language, there are plenty of cognates and familiar conjugation patterns (there are also a few words that are identical to the same word in English, such as "bad", which is pronounced and means the exact same thing in both languages). HOWEVER, since the influence of Arabic is so strong in Farsi, virtually any highly educated Iranian will (1) use Arabic broken plurals for the plural of nouns derived from Arabic and (2) conjugate purely Arabic verbs in the Arabic fashion. It also helps to understand how triliteral roots are expanded in Arabic to make nouns, adjectives, etc., as many of these have been borrowed.

As a result, if one truly wishes to learn Farsi then there is an incredible amount of strict Arabic learning that is incorporated. On the one hand, if you're just interested in learning conversational Farsi, it's just something to keep in mind and the language ends up being easier. However, to really master Farsi requires so much Arabic learning that I would venture to say it's harder than learning Arabic, since in essence one must learn Arabic, and then learn a great deal more.