There is the wrong idea that it's simple to compare KV stores. Actually they are very very different beasts, for instance Redis is not a plain KV store, it can perform even high level stuff for you server side, including sortings, intersections between sets, and so on. So for instance even if you know that the KV store "Foo" is faster and has better coverage than "Bar" you can't just pick "Foo". It's like a programming language. Most KV stores are proposing some kind of data model, not just an implementation of a distributed persistent hash table.

The kind of comparison needed in my opinion is a document explaining the different data model of the different projects, and of course also more general information like speed, test coverage, user base, and so on.