Unfortunately I have never really used Erlang outside of deploying RabbitMQ. I mostly use Go, Rust, Python, sometimes C/C++.
However, Mnesia seems like it is quite a bit more of a complete distributed database engine than Redis. To me the nicest thing about Redis is just the convenience of what it offers: very fast data structures, serialized, optimized (at least by default) for cases where speed is more important than durability. It is simple on many levels and somewhat constrained in scope. Mnesia seems to be aiming more generally in the distributed database category.
So how do you feel they compare?
Really it would be more like Nebulex/Cachex which provide a really nice caching interface across ETS (what Mnesia is built off of) or other data stores.