Assuming that migrating from memcached to redis for the caching you already do is easy enough, I’d go with redis only to keep things simple.
In redis persistence is optional, so you can use it much like memcached if that is what you want. You may even find that making your cache persistent is useful to avoid lots of cache misses after a restart. Expiry is available also – the algorithm is a bit different from memcached, but not enough to matter for most purposes – see http://redis.io/commands/expire for details.