NGC is better than Discovery, but both are pretty sh!tty. History Channel is becoming moreso as well. Both channels used to be so much better, but then they adopted the Mythbusters etc. approach on Disc dumbing down their shows tremendously and the only thing NGC ever shows is stuff about building either an offshore oil rig or a big bridge.
Discovery Science, Discovery Times, and History International are much better channels that haven't dumbed their shows down yet, but nothing beats PBS (Nova, Nature, etc.). PBS >>>>>>>>all.