My take on it
Actually, I think its exactly the same as Spock. But I don't think post-Genesis Spock is the same being as the one who died saving the Enterprise. He made a copy of his katra/soul and uploaded it to McCoy's brain (he obviously didn't do a live transfer as he was still functional after the meld. The original katra died in the Enterprise's Engine Room. The copy was transferred to a cloned body. And even then the copy couldn't have been that good because Spock 2.0 was missing knowledge and had to have his mind "retrained" on Vulcan. Sure, all of his friends viewed him as the same person (as did I when I first saw TSFS), but I now have doubts.
is that what defines continous existence/personhood is quantum entanglement. When you think about it, our own brains are just ensembles of subatomic particles; what makes them constitute a single continuous unit in the first place is their mutual interaction, which in quantum terms creates a correlation or entanglement among them. So if you transfer a person's consciousness in a way that involves a quantum entanglement between the original and the recreation, e.g. quantum teleportation, then it can be argued that there's just as much continuity of self through that transfer as there is between one side of your brain and the other side. This is why I'm able to accept that a person is still the same continuous individual after going through a transporter -- assuming, as I have done in my Trek fiction, that transporters use quantum entanglement the same way theoretical quantum teleportation does.
So if, as I've also assumed in my fiction, telepathy is also a form of quantum entanglement between minds, then a telepathic transfer of consciousness from one mind to another could indeed constitute a transfer of the same original being rather than the creation of a copy. The problem with applying this to Spock's katra, though, is that such a quantum transfer would require destroying the original information (or effectively "moving" it to a new location) and wouldn't allow a copy to exist. As we know, Spock continued to retain his consciousness within his own brain and body for several minutes after he melded with McCoy. Although there is the hypothesis that his meld simply created an "open channel" with McCoy's mind and that the actual transfer didn't occur until the moment of death.
In Data's case, however, it was pretty explicit that what Data downloaded into B-4 was just a copy of his knowledge and memories, not a working simulation of his entire mind. The new Data is an amalgam of those knowledge and memories with the "operating system" of the Soong android, making him a composite being, a hybrid of Data and Soong. He has Data's memories and mostly his personality, but also has aspects of Soong's personality -- including emotion and a greater ease with contractions and informal speech, but maybe including other tendencies as well.