If by "worse" you mean their propensity to violate the rights of Blacks, I would say the answer is no, they are not worse than white cops.
If by "worse" you mean how violent they can get when they DO decide to cross that line, I would say the answer is no, both are equally as brutal when crossing that line.
If by "worse" you mean does it cut into your soul deeper when it's coming from a Black cop, I would say yes.
Getting the shyt kicked out of me by a white cop is far less personal than getting the shyt kicked out of me by a fellow Black person.