I compared both Excel Web App on Office365 and SkyDrive, they differ concerning the available options.
The Excel Web App on Skydrive has an additional ribbon menue "View" allowing to choose whether you are in editing mode or reading mode. In the 'Home' ribbon entry you'll find an entry for sharing the document (Embed to website or give some permissions to other for use the doc). The 'AutoSum' entry is missing in the Excel Web App on Office365. The other functions in the 'Home' menu do not differ (see screenshot below, on the top Office365, unformtunately I was not able to change my language from German to English on the fly)
However, if I have not overseen it, when clicking on f(x) in the Skydrive App, you'll get a dialogue with the available functions, like in 'normal Excel'. This is not the case in Excel Web App on Office365, why ever(?).
On both app, there is a ribbon entry 'Insert' for adding elements. However, as I just discovered myself now, in the Excel App from Office365, there are no entries for adding charts to the sheet. In Excel Web App on Skydrive this is possible (Column charts, Line, Pie, Bar, Area. Scatter and others). Also the f(x) Dialogue can be reached from there in the Skydrive version. The entries in the file menue also differs, as you can see in the second screen below.
I have not tested, if the formula features are different in both versions and I have not tested what happens when uploading a sheet on both versions.
All in all, I am thinking MS is working with different versions of the Excel WebApps on their servers. I am not sure, but I think to remember the version on Office365 corresponds to a previous version of the one on Skydrive.
Please also check out this document about the difference between Excel 2010 and the WebApps.
Hope this helps :-)