Since English and Farsi are both Indo-European languages, one might think English is easier. But Farsi and Arabic have similar systems of writing, and I would guess they've had influence on each other, so maybe Arabic is easier. And I would guess it depends on the dialect. So, if it's possible to say, which is easier?
I am from Iran so I think I can answer your question. but also your question is a hard one. In Iran we start studying English and Arabic both when we are 8 or 9 years old. but how we learn them are different. We try to learn English to speak it. But we learn Arabic for reading Quran. So this way of learning Arabic make it hard to learn the language because the word of Quran is very different as Arab people speak. And you should notice that Arabic has a lots of very different accent. Arabic have a lots of words that are same with Farsi. That's true but also the grammar of it is not same and it is very hard for us. Another important point is that Iran is a big county with people from wide variety of nations so we cannot say one rule for all people. Some Iranians are Arab or work with Arabs(In south and west of Iran). I think I didn't got a clear answer to you but I tried to explain it as much as I could. For me and Who I know (I leave in Tehran, the capital) English is easier but as I said it's not same for every one.