pydata/pandas — pull request #5446 by jtratner (2013-11-06 02:34:52+00:00)
Title: PERF/ENH: Use xlsxwriter by default (and then fall back on openpyxl).

Since xlsxwriter has a smaller install base (because it's newer),
reasonable to assume that if you have it installed, you want xlsxwriter.
Plus, it has better performance characteristics in general (esp. in
terms of memory usage)