i don't think you can really compare Nicholson's Joker to Ledger's Joker. the fundamental differences between them (and their respective movies) is this. the 1989 Batman was like a comic book come to life, and Jack's Joker was in that vein, over the top crazy, comic-book crazy. to me, it seems that Batman Begins/The Dark Knight movies are like, what would happen if Batman was real and not just a comic book hero? what would Batman be like if he were here in the real world? or worse, what would his foes be like? Ledger's Joker was played in that vein, a much more sadistic psychopath. i think they both did great jobs playing their respective Jokers, but even though it's the same character, it's really not, so i don't think comparisons are really valid (even if they are inevitable).
loved the movie tho. man that was a fast 2 1/2 hours, definitely the best Batman movie yet. but it is certainly not for kids. i'm actually surprised this got away with a PG-13 rating.