I've always been told and believed that \ˈī-thər\ (IPA /ˈaɪðər/) is the correct pronunciation, albeit both are indeed common nowadays. From what I am aware, etymologists and linguists believe this was the original pronunciation of the word too. Other contemporary Germanic languages (including the closest modern relatives, Dutch and German), suggest this pronunciation of the first syllable is correct — they have arguably been less altered/bastardised from Old GermanicOld Germanic. Old EnglishOld English (Anglo-Saxon) we know to be an almost purely Germanic language, and thus by simple statistical analysis (as is often employed in historical linguistics) we can be quite confident that this was the historically correct pronunciation.