Redis -> Mongo
@Ninja-Scott I've asked this question, seems to have no exporter for now.
Redis will store your datas in RAM - which means you should have a server with huge RAM if your forum is large. Because of storing db in RAM so the performance is incredible fast. It's faster than mongo.
Mongodb will store your datas in hardisk just like SQL servers but it's faster than SQL because of being the noSQL database. The perfomance is not fast like Redis but when the forum is much more larger you still no need to care about the size of db.
Pretty much as @Hưng-Thành-Nguyễn said, those are the main differences.
Mongo also stores some frequently accessed data in RAM, although I'm not sure of the technical details of it. Performance wise, Redis is faster, though Mongo is no slouch either.