Punch resistance and chin is overrated at heavyweight boxing where there isn't any weight limit after 200 pounds. Punch resistance and chin is relatively even between most boxers at heavyweight with very few exceptions. If a 230+ boxer is landing punches with his full weight, then very few heavyweights would have the chin to take such a punch unscathed without suffering from tremendous damage. Ultimately, I doubt Joshua or Parker have better punch resistance than the other. Both can KO each other if both punch with their entire weight and land their punch on the appropriate area of the other's body. Joshua has so far shown better offensive skills to be able to land such punches more frequently and consistently. Therefore, he is more likely to win by stoppage than vice versa.
Joseph Parker's defense isn't better than Joshua's in my opinion. It's actually one of his weaknesses. He relies on athleticism to make up for his fundamentally lacking defense. Problem is, once he faces opponents who are equally athletic or more, he is likely to be exposed.
Having stated all of those things, I still believe like you that it's close to a 50 / 50 fight in terms of odds of either guy winning. I would make Joshua the slight favorite because his record is slightly better so far. He is also physically stronger. However, Parker does have the abilities to win, most likely by decision rather than by stoppage.

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