Thank you very much.
The only plus of XLSX format is that my iPhone "Files lite" application can
display XLSX files and can not display XLSB.
From you reply it's clear than XLSB format is more functional and more
preferred to use if there are no any conditions to prohibit it.

"JLatham" wrote:

Bottom line is that as long
as all users have Excel 2007, the .xlsb format is probably the most efficient
way to go.