If I had to pick between the two of them, I think I'd pick the Joker, rather than Jason - but really, who says it has to be either of them? Why be so predictable? Why not someone completely different? Black Mask, perhaps - yeah, I know, he's supposed to be dead, but all that talk about masks making you special and how it's revenge 'of one crazy man in a mask on another crazy man in the mask'... I dunno. I'm getting a Black Mask sort of vibe. Sure, there was the 'this is Black Mask's skull that I keep on my desk' moment a little while back, but this is comics. In comics you are alive until proven dead - and that's not always a problem, either.