They both seem to offer the same type of functionality, although from my initial research it seems like SimpleDB has more traditional query options. Any thought from HN readers who have used either, or preferably both, of these products?
Aside from that, CouchDB has views, which add another layer of indirection when reading the data. You break your data up into separate documents, where each doc is a collection of values that should be updated together (atomically). Then you use views to transform your docs to the way your data is displayed. (For example, you could store a blog post and each comment as separate documents, and have a view that returns a post and all its related comments together.) SimpleDB doesn't have this complexity.
I have read a little about the Filter/View system of CouchDB and I didn't really get the point as it just felt overly complex. Your blog/comment example helped though.