Both started out as one-hit-wonders, so it's hard to judge.
Will Smith started out as rap artist and is well known as Will Smith on the Fresh Prince of Bel-air. Since then though, he went big. He can do comedy, action, drama, everything.
Adam Sandler started out as a comedian, was funny throughout his career, and is trying to do different roles now, but really isn't given a chance. He was really good in Reign over me.
In the end i'll give the cookie to Will Smith for bringing me to tears, even in Fresh Prince of Bell-Air:
[ame="
https://www.youtube.com/watch?v=bxulantPiXI"]YouTube- Sad Scene From The Fresh Prince of Bel-Air[/ame]
Hes astounding.