sorry i messed that reply up, what i want to say is are Dwarven or orcish arrows or weapons better that silver ?
They do more physical damage, but they are useless against wisps unless you are using a silver, daedric, or enchanted bow.
Wisps along with ghosts and some other creatures have an immunity to "normal weapons" which is anything besides daedric, silver, and enchanted stuff.
The whole reason behind this is that if you think about it a ghost isn't really a solid object you can strike normally, so the game makers decided that you could hit them, but only if you have certain special weapons.