Andres Valloud performed some benchmarking in the past and his conclusion was that there was a threshold higher than expected when the streamed concatenation was faster and memory savvier. This was years ago, and of course I don't remember such threshold.

However from the code aesthetics I use simple concatenation when I know beforehand there will be two or three semantically equivalent strings to be joined and choose streams when there is an uncertain lenght concatenation�� (most of the times involving appending while iterating).

Not to mention implementations where the stream is passed as an argument, which in my experience are easier to factor.