It depends what you want to do; but basically, Redis is much more performant than CouchDB, and it is much more suitable for a cache system. Redis is an in-memory database (with options to sync to disk), with features for expiring data after a certain time.
Redis is used often for caching, so it may be better in terms of scalability. If you already have Redis running now, then you will have no additional maintenance overhead compared to CouchDB.