You can think Redis as a shared data structure, while Ehcache is a memory block storing serialized data objects. This is the main difference.
Redis as a shared data structure means you can put some predefined data structure (such as String, List, Set etc) in one language and retrieve it in another language. This is useful if your project is multilingual, for example: Java the backend side , and PHP the front side. You can use Redis for a shared cache. But it can only store predefined data structure, you cannot insert any Java objects you want.
If your project is only Java, i.e. not multilingual, Ehcache is a convenient solution.