Well if we are judging them by their complete body of work...I would have to go with Bale.
I love Johnny Depp and it was great seeing him in a serious dramatic role...and he nailed it. Mostly he plays quirky characters though - which are always memorable...but I think a true drama is harder to nail...than say a character like Edward Scissorhands or Jack Sparrow. He does make movies fun to watch - you know his character is going to be entertaining....but they are not serious dramatic roles which I think really test your talent.
Bale - aside from Terminator Salvation which totally disappointed me, he has shown an amazing range and has brought such conviction and believability to his roles....The Batman movies are great movies - but I feel they no way show his true talent. Movies like The Machine, 3:10 to Yuma, Rescue Dawn are great because he was the character - hook, line and sinker. I don't think many could convincingly pull of those roles. I really liked him in Public Enemies...his role was fairly small - but he put on a great accent and even though he was a man of few words - his eyes and slight facial expressions gave a glimpse of what was really going on with this man....he was tormented yet had to hide it.
Now, I would watch any film by either of these actors...no question. It's nice seeing Depp branch out into more serious roles...I don't know if Bale could pull off a lot of what Depp has done and vise versa....they both are extremely good at what they do....so perhaps my vote is a tie.