Since we have already found out the correct pronunciation of either, why not take a stab at how again should be enunciated?
Obviously, there are some words where both of two pronunciations are valid. Take the, for example. The word is usually pronounced /ðə/, but becomes /ði/ if it precedes a vowel or vowel sound.
What about /əˈɡɛn/ versus /əˈɡeɪn/? Is one more valid? Are both valid, but in different circumstances?