Join Date: Dec 2009
Location: Mormon-Town. D:
The question of which special to go for is mostly a matter of metagame, and what you need in the rest of the list. Each has it's pros and it's cons, though there are some good arguments as to why Exec's are the least of the bunch, but in the end, each has a specific job.
Witch Elves are great at carving through basic infantry and high toughness models, but are fragile, and can break against armies that have good armor saves. However, considering the other options, Witch Elves are lucrative in combination with Cold One Knights, as those cover high armor save and are somewhat tough themselves, so you don't have to worry about buffing up only the Witch Elves. And, if you tend to face Skaven, or Empire and such, they'll just carve an absolute bloody hole into the enemy the moment they hit combat.
Black-Guard, as stated, can end up being mediocre, but are notable in that, pretty much no matter what they're up against, they'll always do at least decent. It's a shame about the lower unit size, but in smaller games, that's less of an issue.
Executioner's are the dark sheep of the bunch. I've heard of the ASF horde, and I can see how it would put out a lot of damage, but it is also a *lot* of eggs in one basket. If you have a lot of Chaos or Lizardmen, I'd consider it, but against most opponents, it's a point-sink, and one that's really reliant on one character to allow any sort of damage.
Since there's Cold One Knights, the Executioners seem even less appealing. Try out a Cold One Knight block with BoHG, and a character, and you'll have a more maneuverable and durable unit that isn't reliant on a no-armor-save character to carry the whole unit.