The Dark Swarm list is more durable:
- The Mechanithrall hordes outnumber the Dark Swarm list by more than two to one, but they lack any sort of defense. A Mechanithrall will be hit by most ranged units on sixes or better, and will die on anything but snake eyes. A Bane Thrall unit simply won't get hit by most ranged units at all, and if they face a unit that ignores stealth they feature the same defense as Mechanithralls, and a lot more ARM.
- A opponent with blasts will regularly place the blast where he needs it to be, and inflict a lot of destruction on the tightly packed ARM 12 Mechanithralls (POW6 kills them on average). Against Bane Thralls the blast will nearly always scatter, and even a POW8 blast which are really rare, will need to roll above average to kill a Bane. There are also fewer Bane Thralls on the table, so blasts will hit fewer models as well.
- A Tough check on a Bane Thrall means a lot more than a similar check on a Mechanithrall. Whenever a Mechanithrall manages to live you get a Mechanithrall (woop woop!)... when a Bane Thrall lives you get a pissed of weapon master standing up in his face the following round (even better if that unit has the UA). That means that each Bane Thrall will continue soaking hits until he's dead for good, which can often save other Bane Thralls from a similar fate, as most opponent's will come for the guy with DEF 5 that is automatically hit in melee.
- A Bane Thrall on the charge swings for 27 damage on average, while a Mechanithrall swings for 25.5 with a combo strike, so how can the stealth list hit harder when there are twice as many Mechanithralls? The first thing you'll have to take into consideration is space: In any game featuring infantry hordes you'll muck up charge lanes, and even when you don't you'll be hard pressed to get more than four models charging a heavy on average (some games will let you get more, but we need to look at the big picture). This means that the numerical superiority of Mechanithrall horde means nothing for their force application in any given round, but the extra damage and MAT of the Bane Thralls will mean a lot. The Bane Thralls also bring Tartarus, which allows them to take on Cryx jacks and circle beasts at DEF 13+ and still hit somewhat reliably with MAT8, which is something that should never be underestimated.
- The second thing we have to look at is free strikes: The Mechanithralls are PS 11, and while they hit at PS15 when doing combo strikes, that means nothing on free strikes. This means than a Khador Heavy will take a single point of damage on average per strike, and it honestly won't care... Bane Thralls on the other hand swing for full damage, and taking seven points on a heavy for every Bane Thrall means that the jack is staying where it is. In addition the Bane Thralls have increases ARM and Stealth, making it harder to remove those free strikes by shooting the Bane Thralls to free it up.
- Minor things include offering Terminus a further -2 debuff against ARM, and while this is usually not vital it is very nice to have when facing hard focus campers
- Moving 18-22 models around is easier than moving 38 models around. It also takes a lot less time to do so, allowing for a bit more careful placing of models to avoid clogging up. In addition there's no shuffling around to allow Stitch Thralls to collect corpses, and placing Mechanithralls to prevent someone from shooting your precious Necrosurgeons.
- Having only half the screen allows you to play your Bile Thralls more effectively, since they can now actually get through the screen in order to Purge, and you will lose less of your own screen that way. Now here's the first thing where the Mechanithrall horde has any real advantage, because losing your own Bane Thralls to a purge really hurts. In order to prevent this, I've been practicing setting up the lines to minimize my own casualties, and the one time I did this yesterday I only lost two Bane Thralls and gained eight souls for Terminus... Fair trade in my book.
29-04.2011: I feel a slight update to this post is in order. The Dark Swarm has been found to benefit a lot from one Bane Thrall unit with the Officer & Standard. Originally I was fielding them without the UA, since Terminus grants them Tough anyway, but stationary or knocked down Bane Thralls really mess up the lanes, and the easy solution is to just field the UA. This unit is the one you place in front of Terminus, so come maintenance phase they'll shake whatever was affecting them, and be ready to move out of the way.