Here's a pretty good comparison of the two - http://stackoverflow.com/questions/10558465/memcache-vs-redi....

Disclaimer: I wrote the above article and work for RedisToGo. But that article is unbiased :)

My experience has been that many people are using redis instead of memcached. I haven't switched myself, but it seems some frameworks of various types (ServiceStack, Resque, others) build in a Redis-based caching mechanism, and that's driving most of the adoption for Redis-as-a-cache. Perhaps some of the motivation is that it's easier to kinda-sorta run Redis on Windows than it is to run memcached on Windows.

1 thing I appreciate about memcached is that it is always fast. Redis has a wider variety of features and capabilities, which can come back to bite you if/when the whole app is slow due to some poorly thought out Redis query -- especially likely if you're using Redis to do multiple things, such as (1) store temp cache data and also (2) store user account data.

I ditched memcached for redis a while ago, just because I was already using redis for my queueing, and it made sense to eliminate redundant dependencies.

I'm now using Redis clusters to distribute cache reads so that each webserver has its own local copy of the cache which it reads from, but there's a single master that accepts writes. This does occassionally mean a double-write, but since we're talking about cache data and redis is atomic, this isn't a problem, and it splits the load of my reads, so that I don't end up bottlenecking if the master is being slow.

Memcached has some rough edges (e.g. slab size management) that aren't there in Redis. Redis also gives you a couple extra useful primitives.

I'm not an expert with either redis or Memcached, but in my personal usage, Memcached is functionally at least equal to redis for the things that it does well like LRU caching.

I find myself continuing to use it for those things just to keep my redis config cleaner and optimized for the things redis is (seemingly) better for like persistent storage and message queues.

Redis can be run without persistence, at which point it can be treated as Memcached with extra features (optimistic locking, data structures, etc). Without persistence, performance is often comparable (even with persistence, performance can be comparable, but you're more at the mercy of the IO subsystem).