I've run some benchmarks
Reading from RAM is super fast when compared to Harddisk even on SSD
Regarding your question, redis cache will be always faster than DBs who writes and read from Harddisk. But don't use redis as an alternative to DB, like having a full copy of DB in Redis. It has to be used a cache!
Ram is always costly. What if you have a DB of 100GB? Having a hard disk of 100GB doesn't cost much. But 100GB of RAM is way-way higher.
(There is Redis cluster if you want to scale horizontally)
So if you have thousands of records, cache the ones that are frequently used in Redis.
While developing widgets for MFY, only the widgets that are used within 2 days are stored in Redis. If we didn't get a ping for 2 days it will be automatically purged from the cache. In such a way frequently used widgets will be served from cache. If we need to save all widgets in the cache, that would require too much RAM.
We also use CloudFlare's cache to speed up further. See How I used CloudFlare to reduce API response time to < 100ms