I think it depends on the object you are acting on. If it's a new string each time (you have created a new one with Dim, or assigned a different string from a file), then judging by the results of my brief test, I wouldn't expect to see much difference. If you are manipulating the same string multiple times, then I would expect string builder to be a lot faster.
The bottom line is, test it in the context of what you are doing. Then you will know which is fastest for that particular solution.