A check of the Chessgamesdotcom site shows that its popularity spiked in the 1940s and again in the 1980s and has dropped off since then. According to their database of 6277 games between 1911 and 2015 the percentage results have been:
White wins 42.5
Black wins 28.8
Statistically the Najdorf, based on 24,536 games between 1926 and 2015 the percentage results have been:
White wins 36.4
Black wins 31.9
So, the Najdorf is the better of the two which accounts for its popularity, but there is a lot more theory on the Najdorf that you must be familiar with. Wikipedia has a good general guide to the Dragon. The Dragon looks to still be a pretty good defense for black IF one is willing to spend some time to learn the basic patterns. I Googled “Sicilian Dragon Amazon” and found a some good books that allow you to look inside.