Sorry for the delayed response - I wanted to refer to TLH to pick up some specific points but then I realized that I lent it to a friend and she hasn't returned it yet.
(I still have her entire collection of Transmetropolitan, so I guess we're even...) So I'll have to go on memory.
What makes Dent's shift in TLH more convincing for me is that despite his drive for legal justice, we can see how easily he COULD snap if given the opportunity. ("Two bullets to the head. Couldn't have happened to a nicer guy.") The worst he gets in TDK is torturing Schiff, but he did it for love of Rachel (always the out for a good guy) and we discover later that he wasn't going to kill him anyway. He doesn't truly wish death upon anyone, until his actual snapping point, at which he goes bad completely and suddenly.
Also, even after Dent turns in TLH, you can still see a glimmer of former Dent in Two-Face. One of the most powerful panels in the story, to me, is when Two-Face has a gun aimed at Batman point-blank. Batman reminds Dent of his wife Gilda, and a glimmer of recognition crosses Dent's face. Batman then says, "Give me the gun, Harvey," and Dent refuses. Not by shooting him point-blank, but by hitting him in the face with it. ("Nice try.") Dent acknowledges how far he's fallen, and he refuses to regret it.
I did like the white knight/dark knight contrast, although I felt more for Bruce in that. You can sense how much he really wants Dent to succeed.