StringBuilder Append vs AppendFormat efficiency in C#

Which is more efficient in C#, 1 or 2?

StringBuilder sb = new StringBuilder();
 sb.Append("my string " + myVar + " my string");     // 1
 sb.AppendFormat("my string {0} my string", myVar);  // 2
 

I'm guessing that the question could also be rephrased:

string y = "my string " + myVar + " my string";              // 1
 string x = String.Format("my string {0} my string", myVar);  // 2
 

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