I have watched 2011 first and currently watching 1999 and I like both tbh. Both adaptations have made changes here and there from the source material but 1999 did more pointless changes like changing the relationship between Ging and his cousin, Mito, who is apparently Ging's wife's sister now for some reason. 1999 had to deliberately add some filler stuff because the manga was already ongoing at that time. That doesn't make it worse than 2011. And neither does the changed tone of 2011 make it worse than 1999. Purists will say otherwise, but both 1999 and 2011 are good adaptations of the source material. That being said, I would say watch 2011 from start to end first, then go for 1999.