1) Zack's someone comfortable with himself. He's a ladies' man. He accepts himself for who he is.
2) Cloud doesn't accept himself for who he is. He tries to hide who he is. His mind even takes on the persona of another person to a great extent so that he can even live with himself. None of that's saying a lot for his mental solidarity.
3) Zack doesn't have a reaction JENOVA's Cells.
4) Cloud has a reaction to JENOVA's Cells.
5) It's said that only the strongest may enter SOLDIER. Cloud calls himself weak. How has he displayed weakness? With his mind.
6) Another member of SOLDIER shows only the faintest signs of being affected by JENOVA, suggesting that -- like Zack -- he's got something that seperates him from Cloud.
7) What seperated Zack from Cloud? Pretty obviously it's their acceptance of theirselves. Aerith calls Zack a ladies' man. After asking Cloud if he's ever been on a date, when he replies "No," she says "No, just a mixed-up kid...."
8) Cloud is forced to come to terms with who he is and accept who he is. He then admits the true nature of himself to those closest to him. He has come to terms with who he is, accepting it.
9) All times before his acceptance of himself when his mind was entered by an outside force, his will was either completely subverted (such as when he handed over the Black Materia) or heavily bent (when he nearly crushed Aerith beneath the Buster Sword).
10) After accepting who he was, when the same being who had been bending his will was present in his mind once more, he easily purged them and their influence from within himself.