Both Urdu and Farsi are Indo-Iranian languages. Arabic is a Semitic language and not related to either.

Putting your previous knowledge aside, I think the one that you are most motivated learn would be the easiest since you'll want to put the effort into learning it. Of course, this doesn't help if you are equally motivated to learn both. 

Recently I have read an article which was subdivided languages based on their difficulties. Arabic was considered as the most difficult one along with Japanese and Chinese mandarin, while Persian was in the second order after Portuguese, Spanish.