You're both sort of right and sort of wrong.

Strictly speaking, because of normalization, the weapon speed does not matter. But it's not the dps value that you want to look at, it's the avg base dmg of the weapon. If you have two weapons with the same dps but different speeds, what that means is that the slower one will have a higher base dmg so that's the one you want in your MH. However, if you had two weapons and the faster one also had a higher avg base dmg, you would choose the faster one because again it's the base damage that matters, not the speed.

Off hand of course is the opposite. There you want fast as possible because very few rogue special attacks involve the offhand, the only one that you can do with a sword is Shiv and for Shiv you again want the fastest weapon possible to reduce the energy cost. Higher dps is actually not as important as speed for off hand because the offhand suffers from the damage penalty, isn't involved in your specials, and has the primary purpose of hitting as often as possible to get as many combat potency, poison, other chance on hit procs as possible.

If you still don't get it, then stop worrying about it and just get the coolest looking weapons you can.