All strike-based classes tend to seek the highest damage (applicable) weapon they can get, generally.
Assassination Rogues have access to Mutilate, which does a crap ton of damage and hits with both weapons, but requires daggers. So Assassination rogues generally want the highest damage daggers they can get. But there's an exception here--while they want the slowest MH dagger possible (as long as it doesn't come with a massive loss in stats), they actually want the fastest dagger they can get. This was not the case in Wrath, because they used to be able to buff their off-hand's damage (leading to bigger mutilates).
Now, though, the off-hand is only going to contribute 50% of its damage to Mutilate (so 75%). Of course, if there were no other factors, Rogues would still go slow OH, because bigger Muts = win. But they are also poison specialists. Having a higher speed dagger in the OH leads to more poison procs, and the additional damage from these far exceeds the additional damage to Mut (and everything else) of having a slow OH, especially as they gain mastery.
Combat Rogues are the same--they optimize their weapons for their main attack--Sinister Strike. It only uses the main-hand weapon, so Rogues go for the slowest, hardest-hitting one they can find in order to maximize their damage (their other attacks all scale with AP instead).
Since their OH's max/min damage is irrelevant for their additional attacks, they don't need to worry about going slow--as far as SS is concerned, both are equal. But, of course, other factors exist. First of all, Combat has the talent Combat Potency, which gives each OH attack a 60% chance to regenerate 15 energy. More attacks = more procs, and 15 energy is very appreciable. As if that wasn't enough, we get the same bonus to poison procs as Assassination does (though Combat's poisons do less damage). So combat will always want the fastest dagger they can get in the OH (1.4).
I'm not gonna go into Sub here, because I have no clue what gives the best dps, and none are viable in PvE anyway. But I'm guessing it still heavily, heavily favors daggers, because it lets them use Ambush and Backstab. I know there were talks about hemo-based builds in beta, using slower weapons instead, but daggers get a buff on hemo to even out their damage. All in all, I'm guessing they go slow dagger/fast dagger.
And this concludes the idiggory-just-doesn't-want-to-study lesson!
Anyways, you all are horrible, @#%^ed up people
Never underestimate the healing power of a massive dong.