I think Alley and Curtis both did a good job. The problem is that they were playing two different characters.
Slap the name "Saavik" on them both all you want. But Alley was playing a character who was half Vulcan and half Romulan. Regardless of what made it to the screen, the statement to that effect was filmed, that's how Meyer intended the character, and that's how she was directed. Curtis, by contrast, was directed by Nimoy to be a full Vulcan with none of the hints of emotion that Alley played.
Both are valid choices. However, if Nimoy wanted a totally different characterization than what had been done in TWOK, he should have made it a different character, particularly when you have a totally different actress playing the role. There really was no particular reason the character in TSFS needed to be Saavik. It could have been changed just as easily as Saavik became Valeris in TUC.