well for right now noone can give 100% accurate information on what to use...fact is dual daggers are for pve...u can argue about what u want for PVP because some ppl say single dagger mastery stacks with duel daggers...not sure about that and nobody can give and solid proof to that...so its all rumors..
Yes they can give you solid proof and I said the same thing to you in game. If I take my dagger off and have no weapon equipped and hit dash it can skill crit and I can get 30 seconds on it. The mastery works just like the old skill mastery except they put the description of it into dagger mastery needlessly but it functions seperately, it's not based on your weapon and you won't see differences in skill crit rates with either single or dual daggers, it's the same so if anything it's STR based only.
Rogue doesn't have the burst damage to two shot people anymore, you're reliant around cc, poison and max crit. With dual daggers you're getting stronger pve damage and superior application of poison and max crit levelling in pvp due to the twin hits. I'll be honest and say I haven't bothered to check the +4 crit damage bonus of a single dagger because I'm so content with my duals but dual is 500~ (I'll get an exact figure tomorrow, I'm so tired atm) so it's unlikely that single dagger is far off that.
and ur dash skill might just be a lucky crit out of the random that doesnt proof that it works with dagger skills..and yes as we said ingame..there is still no solid proof that says single dagger masterys works on duel daggers...sofar its just rumor and talk..i gotta test it myself ill have my new duels soon and ill get back with you about results