There simply isn't one solution that will work for all cases.
You really need the backslash version if you have a very long string
WITHOUT newlines that you want to fold across multiple physical lines.
On the other hand it is a pain to terminate every line with \n\ if
you are simply incorporating long paragraphs of preformatted text in
your program -- so that's where triple quotes are best.
String concatenation using the + operator will be costly in certain
cases -- and we can't yet count on an optimizer. There simplay ain't
one and there won't be one for quite a while.
Concatenation through juxtaposition -- well I don't care too much for
it but it's the only solution that lets you indent the strings as you
please without a performance hit.
Finally: I sort of default to doing things the C way unless I have a
significantly better way...
--Guido van Rossum, CWI, Amsterdam <Guido.van.Rossum@cwi.nl>