As an online storage, two of them can be a choice, with officially supported API. Both of them have its advantage and weakness.
Google SpreadSheet supports RSS feed, but has limited no. of rows. GAE Bigtable is more scalable, but harder to be remotely accessed by external party.
Any other comparison? and what's your favorite?
I don't believe that there is a really meaningful comparison between the two. There are too many differences to practically list, and each is designed of implemented to do vastly different tasks.
I think that it is difficult to think up a real-world scenario in which there is a non-obvious choice between the two.
AppEngine's datastore can not be accessed by a third party. It is only available to your GAE application. Period.
I have looked at using Google Spreadsheet to get data out of App Engine. The main reason is that my boss is more comfortable working with things in excel (or looks like excel). I would have it as a simple export option instead of using it as storage.