I thought I would stir the John 1:18 pot a bit....
It seems to me that the place to start in the Theos/Huios debate is with the definite article before monogenes that is omitted in P66 and B, but is present in P75 and A.
Apparently a corrector added the def. articles and rel. pronoun to Aleph.
The Def. art. after Theos strikes me as just plain weird; it isn't needed as a weak rel. pronoun because the rel. pronoun immediately follows it. The def. art. preceding monogenes could easily have been lost by a homoioarchton. My sense is that the articles preceding and following monogenes XXXX are either both 'original,' or neither is original. At this point, I'm leaning towards them both being original per P75.
In scripta continua the two def. articles would read as framing devices forming an inclusio around monogenes XXXX. Why do that unless you want to call attention to the phrase? My guess (and it is just a guess) is that the readers/hearers would have found the phrase challenging. For this reason I think monogenes Theos would have been the more difficult reading to the first or second century ear; it's pretty easy to conceive a monogenes huios, especially in a world populated by the Greek and Roman pantheons, but much harder to conceptualize a monogenes Theos.
my 0.02 (break out the rotten fruit and vegetables and have at it)
2nd year MDiv student at Pittsburgh Theological Seminary