Note if I've gotten any of the maths wrong please do say
Note that for this test I have assumed that all 30 models in the unit have gotten into close combat range. In addition I've not applied any buffs, nor even the standard +1 to attack for being within 8 inches of a leader. These are the bottom-line stats for the unit without any modifiers to make it simpler to compare the two equipment options of Bucklers or Duel Blades.
30 Witch Aelves twinblades (with leader bonus to hit)
To Hit ((4/6*29)+5/6)*3 = 60.49
To Wound 3/6*60.49 = 30.245
30 Witch Aelves twinblades (without leader bonus to hit)
To Hit (4/6*30)*3 = 60
To Wound 3/6*60 = 30
First up note that at this scale of attacks the +1 to hit from the leader is a very marginal difference if any. Of course in real world situations the leader is often going to help out more so than not; however as its only a bonus to hit the effect can be more muted than those leaders which give an extra attack. So don't be too afraid to lose the leader.
30 Witch Aelves with bucklers base attacks
To Hit ((4/6*29)+5/6)*2 = 40.33
To Wound 3/6*40.33 = 20.16
Mortal Wounds generated based on wounds made against Witch Aelves with bucklers
5 wounds: 1/6*5 = 0.83
10 wounds: 1/6*10 = 1.67
20 wounds: 1/6*20 = 3.33
30 wounds: 1/6*30 = 5
40 wounds: 1/6*40 = 6.67
60 wounds: 1/6*60 = 10
Saves difference: Duel Blades (1/6) - Bucklers (2/6)
5 wounds: 0.83 = 1.67
10 wounds: 1.67 = 3.33
20 wounds: 3.33 = 6.67
30 wounds: 5 = 10
40 wounds: 6.67 = 13.33
So what does this all show. Well first up it shows that Witch aelves with duel blades are typically going to generate more attacks than those with bucklers until those with bucklers are taking upward of 60 attacks against them. Of course we have to consider that all the wounds generated by the bucklers are mortal wounds, so they are more guaranteed than regular wounds. However even if you consider them double their value you've still got to take 20 hits, or there about, to generate a number of wounds greater than regular witches with twinblades.
However if we then look at saves we can see that as the number of wounds caused against the unit grows, the number saved from the buckler (compared to if it were duel blades) becomes quite significant.
My take on this is that Witch Aelves with duel blades are ideal for attacking any "softer" target with a 5+ or 6+ save where they expect to deal the majority of the damage in their first round of combat. Furthermore they are best when they will recieve fewer attacks against them - either because the target cannot generate many attacks, and/or because their first wave of attacks removes a significant number of the enemy
Meanwhile Witch Aelves with bucklers are far better to charge at a target with a better save, but which is also capable of making a significant return series of attacks; or to throw them into the pathway of a unit likely to charge, which again has a high number of attacks. Any high attack, low model count unit would be an ideal target for them since the mortal wounds have a greater chance of having a significant effect on the attacker when it is their turn to attack.
They both deal a lot of attacks and have significant potential, but they are slightly different in how they can best be used. Remember the bonus of +1 attack if in range of a leader hasn't been included here, but ideally you should be keeping queens up close to witches for that bonus and for applying buffs.
Sisters of Slaughter.
The stats above also work the same for Sisters of Slaughter, however the big difference is that Sisters of Slaughter have a 2inch reach on 2 of their attacks (whips). This increase their chance of dealing damage as a group since even when bunched up, more of them should always be able to engage compared to if witch aleves had been used. Sisters also have a greater pile range and movement, which gives them a distinct bonus, again, on getting as many of their unit into combat at once.
Again the buckler sisters are more durable and actually want more attacks made against them to show their mortal wound generating potential.
Overall its interesting to see that whilst both bucklers and daggers still keep the units in the general same role, there is enough of a divide that it can be tactically advantageous to use one over the other in specific situations.
Edit: A repeat of the study done this time with 10 man units. Note that the wounds caused against remains the same stats so those can be reused without being repeated
10 Witch Aelves twinblades (with leader bonus to hit)
To Hit ((4/6*9)+5/6)*3 = 20.5
To Wound 3/6*20.5 = 10.25
10 Witch Aelves twinblades (without leader bonus to hit)
To Hit (4/6*10)*3 = 20
To Wound 3/6*20 = 10
10 Witch Aelves with bucklers base attacks
To Hit ((4/6*19)+5/6)*2 = 6.83
To Wound 3/6*6.83 = 3.41
Now here we see something interesting, bucklers are at a significant disadvantage in the number of potential wounds they can cause. Whilst the bucklers are going to keep them alive longer, their damage dealing is distinctly reduced unless they take a lot of hits (to generate more buckler potential mortal wounds). Remembering that that these numbers only go into dealing out damage and don't include the opponent rolling to save, 3 wounds isn't much at all, and taking a lot of wounds to generate potential mortal wounds from the bucklers is a very high risk that the unit could be crippled or wiped out.
Whilst the duel daggers also suffers being weaker at only 10 models, it can at least act more like a glass cannon and dish out at least 10 wounds (more if buffed of course). That increases the chances for them causing the first losses on the enemy instead of them being torn apart.
Whilst at 30 warrior strong units I think there is viability in either build, at 10 man strong (minimum) there is a stronger bias to go for duel blades and treat them as a pure glass cannon - ergo vulnerable but strong if they strike first. Bucklers add to survivability, but at the same time the unit has't got the bodies(wounds) nor save to be a tarpit and hold the enemy, nor is it dishing out much damage to make it a proper threat.