See this page for a more complete discussion:
http://blogs.msdn.com/excel/archive/...20/671995.aspx

Primarily you'll note that the difference is whether or not a file CAN
contain macros or not. An .xlsx file cannot (but .xlsm can) while .xlsb can
contain macros, but of course doesn't have to. Bottom line is that as long
as all users have Excel 2007, the .xlsb format is probably the most efficient
way to go.

"Andrey Son" wrote:

Hello.
New Excel 2007 / Excel:2008 supports new formats - XLSX and XLSB. As noticed
in documentation, XLSB is variation of XLSX but binary format used to store
the date therefore XLSB files are smaller in size and faster to load/save. No
any other differences ware founed by me between this two formats.
My question is - when it's preferably to use XLSX and it's prefer to use
XLSB? It's seems to me XLSB is better by all and it's not needed to use XLSX,
but XLSX was choosed as default Excel 2007 format and this fact must have a
reason. What are the main cases to use XLSX and what are for XLSB?
Than you.