Quick answer ...
Slow Weapon with same DPS means that everytime the weapon hits, it's hits for more. This larger number is what is used to decide how much WW hits for with the OH weapon.
Longer answer ...
However, your rage gen will suffer (how much, and whether it affects you or not is only something you can work out ... your stat may be enough to support it). So if your rage gen is consistent enough when you have a slow OH weapon .. that's the way to go ... no question.
One BIG thing ... NEVER OH a dagger (not anymore)... the reason being that most of your WW damage is going to be calculated from AP. THe calculation decides how much DPS your weapon would do based on 14 AP eqauling 1 DPS. From there, it decides how much damage each hit would do when your OH weapon hits. Now Axes, Swords, Maces and Fist weapons use a normalised weapon speed of 2.4 when this calcualtion happens. Daggers, however, use a 1.7 speed.
Essentially what this means is that every time your OH Dagger hits with WW, you'll be doing significantly less damage, even if your OH Axe, Sword, etc is the same speed as the dagger.
Pre 2.3, off handing a dagger wasn't an issue, because the actual damage of the weapon never mattered, only the dps. Post 2.3, now that your off hand weapon damage is taken into account ... a Slow Axe, Mace, Sword or Fist Weapon is what you want for Maximum effective damage, and a dagger (regardless of speed) is what you don't want.