I haven't done the research myself, but common understanding has been that the damage divided by the delay gives the DPS.
So the Brittle Dagger is ~1.18 DPS and the Rusted Dagger is ~1.33 DPS.
With 2 weapons of approximately equal damage, the faster one is clearly the best choice. When the damage is significantly different though - as in a fast dagger vs. a much slower, much higher damage axe - then the effects of various character abilities may have a significant difference. I don't know a rule of thumb to deal with that, just pointing out that it gets complicated :)
I'd add that, if you're fighting a mob that has high block, parry or dodge / evade chance, then a slow weapon is a nightmare because each hit that is negated by their defensive abilities has a more significant adverse effect on your damage output. If you're swinging more frequently, having some of the attacks blocked isn't such an issue since it's a smaller proportion of the overall DPS.
I also assume that, quicker melee attacks grant a higher success rate of interrupting spell casting?
@Shadowbound
Your point is a perfect example. A fast weapon will more readily meet it's 'average' DPS against a mob that has high evade, while a real powerful but slow weapon will tend toward a much wider variance. i.e. sometimes land several hits and kill quickly, sometime you miss several and take FOREVER to kill the mob.
Also, any ability that gives an affect on the damage of just one hit - like a DL's Life Tap - will benefit from the slow, high DPS weapons. While short term debuffs that perhaps another player puts on a mob might mean that 3 quick hits are easier to get than 2 slow hits. And if you don't get that 2nd slow hit in, your total damage from the debuff is low.
As to your last point, naturally a fast weapon gives you more time to recognize a devastating spell being cast and interrupt it. A separate point is that - to my knowledge - we haven't been given precise info about whether the speed of your weapon improves your likelihood of landing a hit on a quick opponent. It would make real-life sense that it did, but I don't have the resources to test if that is true in the game. If anyone else has, I'm not aware of them releasing that info to the community.