The logic behind leveling Combat Swords until Northend is primarily the poor itemization of fist weapons in the old world as mentioned above. There just aren't enough good fists out there, but there are tons of great swords.
You don't go with a dagger for your main-hand in a combat build because of how the damage of your special attacks (Sinister Strike, Eviscerate primarily) are calculated: based on the average damage (not DPS) of your main hand weapon. Slower weapons tend to have a higher average, hence the advice "slow for combat main-hands."
I found Wound Poison very effective on both weapons while leveling.
Here's a leveling build for your level range. I personally loved Reposte for leveling, but some people hate it. I found it occasionally useful when mobs refused to cooperate with my stun pattern. When soloing you should be stun-locking mobs as much as possible. This will keep your down time to a minimum between pulls. Fewer lunch breaks to get some health means more killing and more XP per hour. For combat that will look like Cheap Shot - SS x 5 - Eviscerate (if they're near death) or Kidney Shot - SS... etc.
And finally, read the sticky. =)
Oh, and to the Mutilate build weapon speed question: Given two weapons of the same DPS and stats but different speeds, go with the faster one for both hands. And, as noted, Mutilate requires daggers in both hands.
Good luck out there!
Edited, Jan 12th 2009 12:46am by TherionSaysWhat