Shocking as it may seem, I don't think there were any depths at all to Ledger's performance as Joker. I don't blame him. The idea of Joker as Terrorist, spouting turn of the century nihilist cliches, was Politically Correct, in the sense of appeasing the current political climate. But it was dumb. Ledger sold dumb with a flashy performance.
Downey created a plausible resemblance to an African-American as represented by a white Method actore and
and Australian. Both personas had to pass muster with audiences who've seen both. Ledger's Joker didn't have to resemble anything on Earth, just hold attention. Technically there's comparison in the challenge I think. Plus, so far as depths go, the sequence when Downey's character breaks character easily had more depth. If the Oscar is awarded for achievement in performance, it's Downey's Oscar. Ledger will probably win, but it will be a backhanded tribute to Brokeback Mountain. But that doesn't mean Gyllenhaal's career is destined to recover.