By taking the comparison into appropriate intuitive scale, RAM is a lot faster than SSD, and of course it is blazingly fast as compared to traditional HDD.
As @a_5mith mentioned above, Redis stores data on the RAM while MongoDB stores them on disk. Most of the server providers (like Digital Ocean, RamNode, AWS EC2) use RAM capacity as the main pricing standard. Disk storage is provided in abundance, beyond what you need (usually). What I mean is that if you are running a NodeBB, it is quite sufficient to run it on a 512MB Digital Ocean instance, and for sure you don't need 20GB SSD disk space if you are running NodeBB alone.
Some application might leverage the best of both (MongoDB and Redis), where in this setup the Redis will act as the caching DB while MongoDB as the main DB. Kinda the same as HDD+SSD hybrid setup. You store the most frequently used stuffs on the SSD for faster access (hot storage), while the less used ones on the HDD (cold storage).
Discouse would be a good example here. It uses Redis as the caching DB and PostgreSQL as the main DB.
Hope this explanation helps!
By the way, to answer your question, it depends. If your system or application does the processing in real time, or you want it to be as fast as possible, Redis it is.