If you want to DPS as an Assassin in PVE go Madness, all other specs are obsolete for it as Madness dose way more damage than the other two specs AND is a lot more fun to play as you have more tools to use and actually have to pay attention to what you are doing and in what order you need to do them.
For instance a Deception assassin:
Stealth<Maul<Voltaic Slash x2<Shock<Dishcharge(Surging Charge) = OK damage.
Madness:
Death Field<Shock(Procs Unearthed Knowledge granting a bonus to damage)<Lightning Charge Discharge<Creeping Terror<Thrash(until Raze Proc)<Activate instant Crushing Darkness Repeat = Insane damage.
I have learned from playing madness that if you play it right meaning by popping everything as they are supposed to be you can keep three dots on your target and just sit there whittle them down with Thrash. My thrash hits for around 1.5k to 1.6k non crit on each hit which is way more than the damage i was doing in deception using voltaic slash.