I would buy both of them to be honest. The second one offers the speed, it has dual bands 2.4/5ghz, it supports all the latest wireless standards, 802.11a/b/g/n.
On the other hand, the AWUS036NH offers 2 watts of power, very handy if you want to get stronger signals. However its support dual bands and supports all the standards, this adapter is not as fast as the AWUS051NH, since it can only go up to 150Mbps.