Good question. Both are very good, not great yet, but one thing separates them in my mind. With Will Smith, it always seems like it's the character playing Will Smith.
I knew Bale was good before The Fighter, but that film convinced me he has what it takes to be great. A great performance.
Smith has been in films I enjoyed too. The Legend of Bagger Vance, Ali, The Pursuit of Happyness, and Seven Pounds. Bale has done more though. American Psycho, The Prestige,The Machinist, 3:10 to Yuma, Public Enemies, and The Fighter. Plus he has the Batman trilogy, which was clearly the best super hero trilogy ever.