Suppose I have a stringbuilder in C# that does this:

StringBuilder sb = new StringBuilder();
string cat = "cat";
sb.Append("the ").Append(cat).(" in the hat");
string s = sb.ToString();

would that be as efficient or any more efficient as having:

string cat = "cat";
string s = String.Format("The {0} in the hat", cat);

If so, why?

EDIT

After some interesting answers, I realised I probably should have been a little clearer in what I was asking. I wasn't so much asking for which was quicker at concatenating a string, but which is quicker at injecting one string into another.

In both cases above I want to inject one or more strings into the middle of a predefined template string.

Sorry for the confusion