I am assuming Redis has all the features of Memcached in addition to its own?
I urge you to read the AWS doco and not assume things 🙂
Your assumption is partially of true, Redis is more feature filled. However they are quite different solutions and you assumption may well get you in trouble if you apply that in a works situation and definitely in an exam.
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You are somewhat correct, based on the overview at this link: https://aws.amazon.com/elasticache/redis-vs-memcached/. However, given that both services have different use cases, it is better to understand what your use case is and determine which services provides the best option for you.
As noted above, they are generally used in different areas. But one important architectural difference is that memcached is multithreaded, while Redis is single threaded. This means that once you start maxing out performance on your instance size, they also have different scaling strategies. memcached can generally benefit from both vertical and horizontal scaling, while Redis can see reducing returns on vertical scaling and generally is a bit more complicated to find the optimal cluster size.