Even with that "bug" Xalek was an amazing tank for me. I leveled sorcerer corruption spec and by time I got him I had the talent for the improved shields so maybe that helped me more.
He held threat off me during force storm and on elites when I would blow cooldowns and trinkets and spam lightning strike and force lightning.
Khem couldn't hold threat off me if I did that.
My vote is for Xalek, he doesn't talk as much as Khem and actually speaks common.