I really think Kirstie Alley's Saavik hada lot more to work with, and therefore made a more memorable performance. As much as I really hated her not being in TSFS, I felt really bad - and still do - for RObin Curtis having to step into the shoes of such a popular and well performed character. She was fighting a losing battle, IMHO.
Kirstie also had the advantage of originating the character. Even if RObin Curtis had played it as Kirstie Alley did, she'd have gotten flack for copying Alley's character instead of making it her own.
On a side note, Robin Curtis was one of the first Trek actors I ever met, and she was an absolute sweetheart, so she'll always have a soft spot with me.