I don't think it's that Cloud is that much weaker, but when he becomes driven, he actually can overpower Sephiroth in spiute of the normal difference in their power. I don't think Seph is nescessarily STUPID either, he just doesn't learn from mistakes... it almost seems as though he doesn't CARE if he loses. Even 2 years after his initial defeat, in Advent Children he refuses to finish Cloud off when he has the chance AGAIN. I mean really, he was up against a wall, and couldn't retaliate, but Seph settled for stabbing him in the shoulder? GO FOR THE JUGULAR DAMMIT! I can't imagine a former first class soldier wouldn't know better. Howver, after coming back from the dead, he probably has an even BIGGER god complex, and the knowledge that his defeat is meaningless.