Redis has more features, and, IME, isn't any slower. So I'd use it instead of Memcached.
Varnish isn't comparable to the other two. Redis and Memcached are used by your application to cache chunks of data, while Varnish uses your application to generate entire web pages and caches them.
Oh this mean i can use Varnish and one of the others... like a frontend (varnish) cache and a backend cache (redis or memcached). Thank you
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