When do you use StringBuilder.AppendLine/string.Format vs. StringBuilder.AppendFormat?

Question

A recent question came up about using String.Format(). Part of my answer included a suggestion to use StringBuilder.AppendLine(string.Format(...)). Jon Skeet suggested this was a bad example and proposed using a combination of AppendLine and AppendFormat.

It occurred to me I've never really settled myself into a "preferred" approach for using these methods. I think I might start using something like the following but am interested to know what other people use as a "best practice":

sbuilder.AppendFormat("{0} line", "First").AppendLine(); sbuilder.AppendFormat("{0} line", "Second").AppendLine();  // as opposed to:  sbuilder.AppendLine( String.Format( "{0} line", "First")); sbuilder.AppendLine( String.Format( "{0} line", "Second")); 

Answer 1

I view AppendFormat followed by AppendLine as not only more readable, but also more performant than calling AppendLine(string.Format(...)).

The latter creates a whole new string and then appends it wholesale into the existing builder. I'm not going to go as far as saying "Why bother using StringBuilder then?" but it does seem a bit against the spirit of StringBuilder.