Well let me explain it like this.
Slower speed weapons have larger white hits. White hits being just ur normal dmg hits. The DPS on the weapon could be shit, but if the weapon speed is 3.4 than that shit is gonna hit hard.
On my rogue, I used scouts blade. In my full blown Ambush gear, it's crit for 300-350 white dmg. I swapped to my Sword of Decay, and now it crit for 500-550 white dmg.
Why this weapon is so good is not just because of the white dmg increase. When you use a very fast offhand weapon, your mongoose proc rate shoots way up. Where as a prot warrior would only have the increase of white dmg.
For rogues, I would imagine sword of decay being BiS, for warriors; it could go either way.
Larger white dmg with some stat deductions(stam/crit/hit/str/etc) or less white dmg but stat increase. You don't gain any kind of special hidden buff aside from the increase of white dmg from using a sword of decay.
if I were you, i'd use the 7/7 wep or the 5/7/hit wep.