Actually, these two are not the same, in particular the spacing is
very
different. I ran into this problem just a few days ago: With {...},
when
lilypond decides on the space between the words, the spaces between
the words
was a too tight, so it was hard to tell words apart and read the text.
With "...", the width of the spaces seems to be taken from the font
and the
text is much more readable...
As an example, look at the attached test case, where the second
title line
(using {...} for the markup) is way too tighly spaced and thus hard
to read.
I had an even worse case, but I can't remember or reproduce now.