I forgot to snag a picture, but I got what I would initially consider a great roll. x2 Crit Chance, x1 Crit Damage, Damage (all of these being blue or orange) and then a white durability.

Usually I draw my eyes to the Crit Chance AND Damage and go "oh great! this seems awesome!" however, I then thought about it, and am curious as to how useful this actually is on the Affliction DoT from the trap:
Does crit chance or damage matter for Affliction? If I get a crit, are all ticks of affliction crits as well? Or rather, does each tick have a chance to crit (equal to the chance of the trap? OR is only the first initial hit affected by crit

So the long and short: Is Crit good on the gas trap? Or would I have been better off with flat damage here, and should I not bother upgrading? Thanks!