Kael in my opinion.

I run with a relentless set, high speed and attack. I usually clear the stage in 10-15seconds without the enemy ever getting a chance to move. 

Actually, for late game, you probably won't be using either one anymore - they are rares after all, so at some point you'd probably replace them with epics or legendaries, the only rare who will probably never be replaced on your teams is Coldheart. The real difference between Kael and Elhain is that in the early and mid game, Kael is good everywhere and at least decent on Clan Boss because of his poisons (though the weak poisons on his A1 make him merely decent - sooner or later you'll still want to replace him by a poisoner who doesn't put any weak poisons, like Frozen Banshee). Elhain doesn't have poison or any other utility other than doing solid damage, so she's fairly useless on Clan Boss and also less good on some dungeons like Dragon. 

I'm not sure what the answer to your actual question is, whether Kael or Elhain is the most efficient at farming brutal 12.6, but for sure both are able to do it relatively smoothly. So I'd recommend you to pick Kael because he will offer more in other parts of the game. Not all the way through endgame, but still for quite a long time. 

Kael until I got Skullcrown