TDNT goes with "only begotten" in John. That work has been accused of depending too much on etymology, but in this case I don't think it does. I agree with "only begotten" in John, and we used a Japanese equivalent for that in the Lifeline Japanese New Testament, 唯一生まれたご子息.
Back in the 1950's-1960's there was some controversy about the meaning of monogenes, and conservatives went with "only begotton" in those days. Maybe because of that, ever since I learned that some modern versions went with "unique," I've felt that was a weak word to use for Christ. We are all unique in some way or other. To simply say Christ was unique doesn't tell us how He was unique, but "only begotten of the Father" does specify His uniqueness.