[derefr]

> assuming that people are in fact paying at least $6k

That's the big difference. UBI most helps the people who are paying the least tax right now, such that a pure deduction wouldn't do anything for them. It's a "negative regressive" tax, whereas an increase in deductions/decrease in marginal rates—or a tax credit—is a "negative progressive" tax.

[dragonwriter]

A tax credit, if it is refundable, is exactly the same as UBI, and even if nonrefundable isn't “negative progressive” in the sense that a deduction is.