In general, you want a build that focuses on one aspect of the game. With 4 role slots, it's very easy to have a PvE and a PvP Nightblade, with 2 more slots still open.
For a PvP build, I'd recommend something like this.
You'll get the always useful Combat Survival from Saboteur, and a 10 sec CD on Adhesive Bomb. Taking Riftstalker to 16 gets you 15% damage and crit for 10 sec after any phase shift, as well as the shift that removes stuns, roots and snares. This is my current Nightblade pvp build, and it works very well for me. Lots of mobility, slowing and survival, and you still do great damage too.
I don't currently have a Nightblade PvE build, but if I did, I'd probably use this build.
You get the full 51 in Nightblade for max single and aoe damage, 5% crit from assassin, and the extra hit and auto attack damage from bladedancer.
As for the Infiltrator soul, I usually don't bother with it. Assassin has better stealth options and burst damage, riftstalker has far superior mobility and survival. It's not that it's a bad soul, just that for the same amount of points, you can usually get more from one of the other rogue souls. It doesn't help that most of the damage buffs on the left side deal with physical melee attacks while the Nightblade is a ranged elemental damager, and the right side suffers from the Riftstalker just being so much better in terms of survival.