Knives are much better for training, especially on Yaks, great xp per hour. I would even say knives are better for pking, the only time I used darts was whilst ranged staking, they were better than knives imo.
they are actually the same speed pretty much in the end BUT
What alot of people dont know is that iron darts are same bonus as bronze knife, steel dart same as iron knife, ect. so for example when i was getting 99 range i used steel darts because they were really cheap (cheaper than bronze knives) ofcourse people know about this now but its something to look into to see if you save money by using 1 higher metal of dart.