I'm trying out a squad of Helverins as fire support to replace some NDKs since they're about the same points. 4d3 S7 -2 d3 shots is waaaay better than any shooting in our book, and they can sit on a back objective and still contribute to the killing, unlike GKSS or servitors
I ran a calculator to figure out how the Dreadknight weapon options compare vs bigger targets. This is with the Surge of Wrath re-roll to hit/wound, but I didn't factor in the reroll damage:
Vs T7, 3+ Save
Hammer: 13.67 wounds
Sword: 9.22 wounds
Vs T8, 3+ Save
Hammer: 12.5 wounds
Sword: 9.22 wounds
Vs T10, 3+ Save
Hammer: 12.5 wounds
Sword: 7.78 wounds
Vs T11-13, 3+ Save
Hammer: 12.5 Wounds
Sword: 5.76 wounds
Vs T14, 3+ Save
Hammer: 10.55 wounds
Sword: 5.76 wounds
So the Hammer is clearly ahead, and sits nicely around 12-13 wounds on average. Combined with the reroll to damage I'd say it will stand a good change of killing most tanks in one round, compared to 2 with the sword. Invulns tend to drag things back towards the lower AP sword, and a 2+ save further towards the hammer, as you'd expect. It seems there is a big enough difference between the two to warrant picking the hammer for tougher targets, and it combos well with his re-roll ability. The Hammer also gives him either 4 or 6 extra dice if you want to use the Tank shock stratagem too, which is worth considering. So I might actually be leaning towards the Hammer at this point. Just gotta keep him away from hordes with a Gatling psilencer and Flamer perhaps.