As with most weapons in the world of Runescape each has there place and there purpose. Enchanted Dragon Bolts are very good against low defense players because the max hit from spec is based upon your opponents defense. The highest recorded spec from Dragon (e) is 820. Onyx (e) works well against players with high defense or players that are wielding an anti-fire shield or dragonfire shield. Also a super anti-fire potion will negate the spec from dragon (e). Onyx (e) bolts have a chance to deal some extra damage. You are healed for a portion of that extra damage.
So Dragon (e) are better against very low to medium defense pures (1-45 def)
Onyx (e) are good against high to very high defense players (60-99 def)
Spec not considered both bolts hit about the same with a ranged strength difference of only 3. Onyx being the highest ranged strength ammunition for a crossbow at 120. The highest ranged strength ammunition in the game is Cannon Shot coming in at 150.
Hope this helps.
