Well I would have to say tentatively Cloud is the stronger of the two. This is only because he does defeat Sephiroth in the end one on one after fighting SIN Bahamut and the Remnants. However there are some points that are difficult to compare such as whether or not Zack's bullet time is superior and before people start complaining about it there is a scene where Cloud is actively deflecting bullets in Advent Children rather than just holding the sword up. The difficulty in the comparison is that Zack was dodging many bullets from many gunmen where Cloud was deflecting a single gunman. However because Cloud also has a disadvantage in that he was on a motorcycle at the time and did not have his full range of motion to dodge. I am going to say the active deflection of bullets as in not just holding the massive sheet of metal known as the buster sword in front of him is good enough to give him a close in the gap of speed between him and Zack.
The actual moment Cloud becomes stronger would probably be Advent Children when he is cured of the geostigma.
The actual moment Cloud becomes stronger would probably be Advent Children when he is cured of the geostigma.