They are not identical. Memcache is older but it has some limitations. I was using just fine in my application until I realized you can’t store literal FALSE in cache. Value FALSE returned from the cache is the same as FALSE returned when a value is not found in the cache. There is no way to check which is which. Memcached has additional method (among others) Memcached::getResultCode that will tell you whether key was found.
Because of this limitation I switched to storing empty arrays instead of FALSE in cache. I am still using Memcache, but I just wanted to put this info out there for people who are deciding.