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Excel Web Apps is more compatible with Excel workbooks and they upload with less errors. However, Google Docs still provides greater functionality, although more conversion work is required. Microsoft's comparison is therefore somewhat one sided.
Excel Web Apps does not currently have Validation - a key feature for me. Google Docs does.
I havent' check Excel Web Apps' use of array formulas yet but that is another big plus for Google Docs, although you need to convert them to Google's functions.
Perhaps Excel Web Apps will develop more functionality. As it stands, it's possibly a better solution for basic workbooks but Google works better with advanced functionality.
1 year, 8 months ago on Microsoft Excel Web Apps vs Google Docs