Whew, that's very very interesting and I feel like I barely know enough yet about Tolkien to really comment intelligently... but then when has that ever stopped me before? LOL So, take this for what it's worth from someone who's not read and studied Tolkien for as long as the rest of you:

I've always seen Denethor as weak rather than evil. I do wonder sometimes if pride isn't often a false front erected to conceal insecurity that one is not willing to confront in oneself. If that's the case with Denethor, that to me could be the crux in the difference between the two characters: Aragorn seemed able to confront the insecurities he felt about himself (thinking of the way he questions his ability to make the right choices after Gandalf's fall, for example) and overcome them. Denethor, however, seems not to have possessed that ability and instead cloaked himself in pride. And of course, the whole bit with the palantir and Sauron plays into that. I get the idea, and correct me if I'm wrong, that Sauron was an expert in delving out the weakness in the minds of those he desired under his control, and in Denethor, he found a fertile field of insecurity to manipulate and corrupt. And I hope that makes some sort of sense!