This is a curiosity of mine. Each time I attend a concert, I look at the double basses to see whether they have either a C extension, 5 strings, or neither. For the concerts that I have attended in the UK, the answer seems to be mostly C extension. However, on TV or discs, I see 5 strings occasionally; I think (not claim) that this seems to be more common in Germany.
What are the pros and cons?
Is the extra string always C to just enable an octave below the cello or is it sometimes B to retain the spacing?
Also what music requires it or may use it? I have not noticed notes below the standard 4 string bass being used in concerts that I have attended but, of course, my musical tastes and powers of observation might be the explanation.
Answers on 5 strings bass guitars are also welcome. I get the impression that the extra string is more commonly B on these.