depends on your skill set really. imo, every dagger should be 15^50, extra dmg is definitly an assassin+. +5 energy is...meh, i guess good in a way, but you need to think fo some things first. sin's attack very fast, and they have critical strikes. 8 points into critical strike will land you 2 energy every crit. Also, if you have a zealous mod, you are getting +1 every hit as well, so basically you can have a chance to score 3 energy every hit, and at the rate the assassin attacks, your energy bar will be full quite often. The best way to explain how I choose what set of daggers to you is by first, telling you the two sets i have, and let you go from there. also, keep in mind that my critical strikes are always at 8 or higher, so i have the chance for at least +2 energy on a crit.
Primary set: Zealous Golden Talons of Fortitude
15^50
+1 -1
+30
Secondary set: Sundering Dragon Scythes of Fortitude
15^50
20/20
+30
Primary set: I use these if I am going to be using any enchantments that take away from my energy degen (ex. Aura of Displacement or Recall). Yes, you do lose one energy degen from the daggers, then another from the enchantment, but the +1 energy every hit is extremely helpful do to fast attack speed, as well as a decently high chance to crit landing you +2 more energy is excellent. My main build runs AOD, so I usually stick with my Talons.
Secondary set: I use these when I am running a build that does not run an enchantment. There are some areas of the game where you just can't use enchantments because of the high chance to get them stripped. I usually use death's charge in these areas, and keep moebius strike as my elite. Without the worry of degen, extra damage is excellent. All of my dagger attacks cost 5 energy, so the +2 energy from my crit chance keeps my energy reserves where they need to be. In these cases, I go with my Scythes.
Of course, this is what I have found that works best for me. You may see things another way, as well as another person too. Just give it a try for yourself, and see what you think.