In principle there's no difference between a negative income tax and UBI funded with progressive taxation. Either way, some people pay more than they get, and some people get more than they pay. The net cost as a function of income can look identical for each.
What I mean is that negative income tax is just a way of implementing a particular kind of UBI. It's not a distinct thing. In what way are the incentives different? You must be referring to a particular kind of UBI which is distinct from negative income tax.
Is the incentive to work different? I thought it worked like this:
Under UBI if you earn $1 you are always $1 richer. Under negative income tax, if you earn under some threshold, when you earn $1 you are < $1 richer because the govt will hand you less.