If they had been able to get Alley back then it would have made the movie much better, I always liked her version of the character, Curtis just seemed to be there but that could be down to Nimoy Vulcanising the character where as Meyer wanted her more emotional than Spock.
It was not the difference in actresses, it was the difference in directors. Meyer originally directed Alley with the idea that Saavik was half-Vulcan/half-Romulan. The scenes that stated that were ultimately left on the cutting room floor, but that's what his direction was based on.
Nimoy then directed Curis as a full Vulcan with virtually no expression of emotion what-so-ever. Which is fine, except that since Saavik the character had already been established in TWOK, it seems a bad choice to make such a drastic character change, particularly when you're already changing actresses.