Long story short, slower weapons are generally superior to faster weapons (with equal damage ratings) because of proc rate normalization.
The proc rate you see on inspection (such as 5% on imbued weapons) is actually normalized against a 3 second interval. Every 3 seconds, the weapon has that chance of having fired its proc. Weapon #1 with 2.0 delay and a 12% rate actually has a 8% chance of firing every swing. Weapon #2 with a 4.0 delay and that same 12% rate actually has a 16% chance of firing every swing. This was done to make slow and fast weapons equally desirable, and would work fine, if all people did was autoattack.
The (probably) unintended side effect is that other offensively triggered procs or proc buffs you have are affected by this normalization. If you use combat arts which count as melee damage, and can trigger procs, the rate for those procs will be affected by your weapon speed. If, for example, you are in your offensive stance, swinging Weapon #1, you've got half the chance of firing Furious Assault on every swing versus what you would have with Weapon #2. Each combat art you use will refer to those same rates when checking whether Furious Assault fires or not.
Now consider what happens when you're using multiple proc buffs, in tandem with AE melee combat arts, such as Assault or Rampage. Each of the hits from these multi-hit CAs benefits from increased proc rates.
All of this has been observed and debated quite a bit since last November. There are some who insist that DW are a more potent choice, and that is generally true at lower level ranges where there are more quality DW weapons available.
Now, that said, there are pending changes on Test which seem to be aimed at diminishing the advantage that slow weapons have over fast weapons. First, DW damage output is getting boosted by some unknown amount. Secondly, and more interestingly, riposte rates are being adjusted so that faster weapons do not result in as many ripostes as they do today. I haven't seen any conclusions drawn from anyone testing this, but it's possible that you may see tanks switch to faster weapons as a result.
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