Excel Viewer does exactly what its name says: opens, displays and prints Excel documents even if you don’t have an Excel processor installed. The drawback comes from the fact that users can’t edit, create or save Excel workbooks.
On the other hand, if the document is found online on a Live SkyDrive, then the equivalent Excel web app will spare you the effort of downloading the file to a computer. Permission given, the respective document can be modified within Excel Web App.
As for the offline version, we have to mention that it has the ability to open all types of Excel files and macro-enabled files.