Hi Monique! It sort of depends if you mean Modern Greek or Ancient Greek, as well as how broad you want your Arabic knowledge to be since there are many different dialects of Arabic. Greek and English come from the same language family: Indo-European. This means that the two languages share a lot of vocabulary and grammar. Arabic, however, is not at all related to English because it comes from a totally different language family. So I would say that Greek is the easier language for an English speaker to learn.
