> The map would be preferred due to readability, performance, or both but more
one than the other?


Primarily because if you've got a tool designed for the job, using something
else feels clumsy.

Performance isn't going to be significantly different if (a) you use key(),
(b) the data is of modest size, (c) both the key and the value are strings (if
conversion to or from strings is involved, this can be a very substantial
overhead.)

Michael Kay
Saxonica