your idea and mine are basically two ways down the same road. But from what you've told us, I'd say the difference between Skullsmash and Aura of Pain would be that Skullsmash is utility-based (It shows you are a dedicated thiefplayer.) while Aura of Pain would be more offensive-based as it uses would only become obvious in situations that involve direct and open combat situations.

I'd say that Skullsmash too seems to be well within the mindset of the Anti-Paladin class and would definately help with giving an A-P a more reliable first strike capability than the Sleep spell does. On top of that, it would also be helpful in certain situations where the A-P finds himself/herself outnumbered, unlike the Sleep spell which is too unreliable to make the attempt at such tactics worthwile. All in all, I think Skullsmash wouldn't be unlike an Anti-Paladin version of the Blackjack skill really.

The problem I see with this however is that this second aspect of Skullsmash, namely the ability to knock out players and thus giving an Anti-Paladin the potential to scatter an opposing gang, collides greatly with the reason Anti-Paladins were given the Teleport spell. Its uses and its effects seem a bit too subtle for a class which firmly stands with one leg into the brute physical departement. The utility/support/cunning aspect of the Anti-Paladin should in my opinion mostly come from the magic spells, covering everything from protection to means of escape to the ability to cause direct extra damage to the opponent. The sheer brutality and psychotic view on life of the Anti-Paladin class are best translated to the outside world by their customized and unique physical skills, such as Lashes of the Slave, Faceslash, Crippling Strike and Vault.

That's why I would prefer Aura of Pain myself. It can greatly add to the insidious black magic part of the Anti-Paladin's arsenal, and it would also make them a little bit more dangerous and resilient in direct physical combat, as a player would expect from a knight-type class. And if need be, the advantages of the spell can be compensated for in a great many ways as the IMMs would see fit as they begin considering the balance issues.

That Skullsmash idea sounds like a pretty damn interesting something for the high(er) ranks of the Push path though. But that's off-topic, I guess.