Testing with iptables -j DROP suits better when testing "host down" scenario, as DROP drops packects silently. In case of stopped memcached linux usually will generate tcp rst on connecting to its closed port immidiately, so you won't get real socket timeout, as with -j REJECT iptables rule. In real scenario there may be even a bigger timeout, for example because of ARP.
Testing with iptables -j DROP suits better when testing "host down" scenario, as DROP drops packects silently. In case of stopped memcached linux usually will generate tcp rst on connecting to its closed port immidiately, so you won't get real socket timeout, as with -j REJECT iptables rule. In real scenario there may be even a bigger timeout, for example because of ARP.