Same actress, in both she kicks a**... literally. Both movies have a lot of martial arts, and the leading lady fighting people to help someone.
One is a Korean movie and the other a Thai drama, but both have a hitman who falls in love with an innocent woman who's an artist. (And both are awesome.)
The newer Let Ratree is a remake, the basic plotline is the same while some things were changed.
An Assassin and Daisy both have an interesting story line about an assassin and a girl, but otherwise they are completely different. (I loved both, though.)