Ever since I saw him on that odd, Starfire-focused future episode of Teen Titans I've always thought Nightwing was pretty cool. He was grown up Robin, but with a bit more of an edge to him. He had something else to prove. He had his own city, Blüdhaven (Gotham's sister city) and his own new, fresh set of rogues that he had to take on. I never got to read much of Nightwing, so I only ever knew about him as a side character going on quests with Batman, or fighting for a cape against Jason Todd.
Dick Grayson, though, I know a little more about. How he was Robin, then Nightwing, then even Batman. In comics, there are few who can say they've eclipsed the master (Bucky Barnes and Sam Wilson come to mind immediately). But Dick Grayson has a little more going for him, and that, I think, is in part due to the relaunch of the New 52 in 2011.