Well if you are using Inno DB only and have limited RAM, Inno DB buffer pool without a doubt should get first priority.
If you have some spare RAM, it is highly recommended to use query_cache for Word Press sites.
Value of YES only indicates that the support for My SQL Query Cache is compiled into the binary but does NOT indicate that it is operational.
So the only way to actually determine if query cache is operational is to check if queries are being inserted and retrieved from the cache.
The Second Level Cache is designed to reduce the amount of necessary database access.
It sits between your application and the database to avoid the number of database hits as much as possible.
There are various types of caching mechanisms available.
The Query Caching is one of the caching mechanism used to improve performance.
It is also very worth mentioning that My SQL uses byte by byte comparison of SQL statements for the cache.
This in practice means that these two SQL queries will be considered different: 'SELECT * FROM users' and 'select * from users'.
Caching Regions are specific region into the cache provider that might store entities, collection or queries.