Nightwing has a much stronger conscience, meaning he will think about doing things before he does them. Though they are both strong fighters, Red Hood is more brutal, and quicker to respond, with much more violence -- almost like Damien, a character that I despise. Damien is a bit brainwashed by the cult he grew up in, and looks down as good guys, calling them "protectors," like it's sound kind of weakness.
Red Hood is more of a good guy, meaning he isn't like Damien who is willing to go after good guys... the Red Hood is brutal against bad guys -- not showing the kind of mercy and consideration that Batman extends to them. Your answer is Red Hood, because he wouldn't hold himself back, while Nightwing would hold himself back, as he is more like Batman in this area -- perhaps even moreso. The fact that he holds back doesn't mean he is less strong, or less skilled - he just has a different worldview, that affects his choices differently. The Red Hood was a hood kid (or street kid) who didn't grow up with a loving family, but had to adapt to the Gotham slums, and deal with corrupt cops. He's more independent even as a young kid, being less susceptible to being molded as a child like Richard Grayson, whose views would be more dependent on a parent figure. Given the negative interactions with corrupt police, the Red Hood wouldn't look at the world with rose-colored glasses, as would be more of the view of Richard Grayson - even when his parents were killed, he didn't have to live in the slums and fend for himself.
Even Damien thought of Richard "Dick" Grayson as naive, and looked down on him for it. Grayson's innocence isn't to be despised though, as it gives him a good nature - but his lack of experience in dealing with the extremely dark elements of life makes him viewed as less respectable, by those who have that awareness.