This is really a reply to this post on Moosie's blog that I couldn't fit into his comment limit. If you're not interested in Batman and/or Vader, don't bother. :)
Moosie, while I enjoy (and agree with) your comparisons between the two Dark Knights, I feel there are several other factors we have to consider.
Firstly, Anakin Skywalker's talent is a gift; he never had to earn his abilities, and boy does he know he has some abilities. As a kid, he won his first pod race, then considered impossible for a human being. Yet because he never actually had to earn his skills, because he was just so naturally good at everything, it made him proud. It made him wonder why he had to follow all these dumb teachings when he knew everything already anyway. And pride doth come before a fall.
Batman, on the other hand, had to earn everything. Granted, he was incredibly rich, and incredibly brilliant, but all his talent, all his ability, all his skill, they all boiled down to one thing: training. This guy never had any midichlorians; yet he could jump higher, move faster, hit harder than nearly anyone he cared to face, and only because he earned it. Every. Single. Punch. His only real talents are his brilliant mind, and stubborn determination. And earning his abilities never made him proud. It never made him think "oh yeah, I'm so badass; I could take on Master Yoda anyday". It only served to make him realise that no matter how hard he trained, how good he got, there would always be someone who was better. Because he's just an ordinary man, after all.
Secondly, their situations are reversed: Anakin has try his hardest to be the goody-two-shoes Jedi Knight all day. And we love him because he does try. But that's not the real him, and it will never be. The real him is proud, and reckless, and even stupid at times, and he has to hide this guy deep within himself, because that is not the way of the Jedi.
Batman's the total opposite (and I'm going to draw more from the comics here). Batman is the real him; it's Bruce Wayne that's the facade. The night is a release for this guy, a time when he can remove his public mask and be who he really is: the Dark Knight, the protector of his city. And make no mistake, it is his city. But most importantly, Batman doesn't pretend to be Bruce Wayne because he has his Jedi brothers looking over his shoulder, because he tries to believe that this isn't the way a costumed hero is supposed to behave; Batman takes on his Bruce Wayne identity because it draws suspicion away from himself. It is a concious choice he's made, one that serves his purpose and no one else. Sure, he doesn't like it, but he's a brilliant guy, and he knows it's the best way.
Thirdly, is the difference in terms of the support they receive. Obi-wan tries his very best to teach Anakin the ways of the Jedi, but he can't be there all the time. In fact, he's practically never there whenever Anakin faces a critical decision. He wasn't there on Tatooine. He wasn't there in Palpatine's chambers. He just wasn't there. And this in no way means he's a poor mentor, it's just the demands of the circumstance. They are fighting a war, after all.
(Drawing from the comics again) Batman, on the other hand, has an entire posse. Has he never felt tempted to go over the edge? Sure he has. But his will is strong, and his principles are firm. And everytime, anytime he's even come close to doing the wrong thing, there's always been someone there for him, be it Alfred, Jim Gordon or Dick, or more recently Oracle, Robin and Batgirl. Someone's always been there to give him that little tap he needs to remind himself why he's dressed up as a big black bat when it isn't Holloween.
I know the post is really about the two movies, but this is just my two cents about the characters as I understand them. :)
Edit: Gah, no idea why I typed "Solo" instead of "Skywalker". :-P
Good catch, Dave. ;)