For what it's worth, although I like Alley, I think Curtis had more of a Vulcan quality to her. And she played the character the two previous times we saw her.
I thought it was Gene Rodenberry who thought Saviik wouldn't betray them. Nick Meyer thought he knew the character better though as he created her.
One of the flaws of VI to me is that once you know there's a traitor on board, it's not surprising that it's the new girl. The character is invented for the movie for the sole purpose of being a villain, which I find unfortunate. I'd call it lazy writing if I didn't know it was Roddenberry's fault.