I agree with uOrochimaru. Kabuto copied Orochimaru in order to even make anything out of himself and have an identity. It's only thanks to Itachi's Izanami that he feels that he can be his own person at this point and no longer has to be like Oro or be his tool.
'Stronger than hiruzen' means to me is that Orochimaru can heal itself and still alive.What a joke 'oro is stronger than Hiruzen?'...
story wise kabuto is definitly better, we got a good backstory and a real character development, from the one who co-started this war to the one that saved one of the mains char.Orochimaru. Kabuto copied Orochimaru in order to even make anything out of himself and have an identity. It's only thanks to Itachi's Izanami that he feels that he can be his own person at this point and no longer has to be like Oro or be his tool.