Error Handling: String VS StringBuilder

I am trying to output a format string to display. Then I came across an error. Hope you guys can advoid it.

As you may all know StringBuilder has performance advantages over complex string.
So what's the different between
String.Format
and
StringBuilder.AppendFormat

I've found it if the args number is not correct in string builder it will throw the following exception while string will not.

Error: {"Index (zero based) must be greater than or equal to zero and less than the size of the argument list." }



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Cheers,

Gary