Well it pretty much comes down to what you want from your companion, do you want him/her to be a tank (which ashara isn't, but khem is) or rather have a damage dealer (which ashara is and khem wants to be). I personally went with ashara, but im a tanking assassin so i needed the dps and so far, im not disappointed with her, she can be really deadly with proper gear (like any companion).
hopefully this helped abit