Executes what? If you are doing a lot of concatenation, go with a stringbuffer. It doesn't reference new memory every time you reassign it like a string does.
Laugh, and the world laughs with you. Cry, and you just water down your vodka.
For concatenation i would think that using a StringBuffer right from the start would allow your code to execute faster since the compiler uses StringBuffers to implement the string concatenation operator.