Instead of asking us, you really should have done your own research. It only takes about ten minutes to find out which one's better.
Let's begin. I'll only be using the (p++) items for this.
Code:
[B]Rune Knives (p++)[/B]
+25 Ranged Attack Bonus
+24 Ranged Strength Bonus
Speed: 7 (http://runescape.wikia.com/wiki/Rune_knife)
Price: 3,934 (http://services.runescape.com/m=itemdb_rs/Rune_knife_%28p++%29/viewitem.ws?obj=5667)
Code:
[B]Dragon Darts (p++)[/B]
+18 Ranged Attack Bonus
+20 Ranged Strength Bonus
Speed: 7 (http://runescape.wikia.com/wiki/Dragon_dart)
Price: 1,883 (http://services.runescape.com/m=itemdb_rs/Dragon_dart_%28p++%29/viewitem.ws?obj=11234)
I'm sure you've noticed the differences between them already. There's a small, yet significant increase in bonuses when you use a Rune Knife over a Dragon Dart. If you want maximum effectiveness in PvP, Rune Knives are the way to go.
Another difference is the price. The Dragon Dart's bonuses are quite comparable to a Rune Knife, but still slightly worse. But the difference in price makes Dragon Darts a better option for those with low budgets or for people who "just don't feel like risking too much".
Conclusion? Rune Knives are better than Dragon Darts, but are much more expensive, especially when bought in bulks. Which one you buy, is entirely up to you now.