Hello!
Even though the +3 on cobra rod may sound great, the onslaught chance is only 1,7%. This means that you'll need approximately 2 minutes to get one proc with 60% more damage. This damage could be any kind of hit, including the rod hit, so maybe you won't get high damage.
Soulhexer, on the other hand you'll get more ML, mana leech (which is greatly needed) and ice protection, so you'll get much more stable damage. Because of that, I'd rather much more have a soulhexer than a cobra rod tier 3.