Short answer is: yes, it would have been a better story if Saavik had been the one to betray the crew.
Agreed. I would have hated to see Saavik turn traitor, and I'd never have suspected her - which is why it would have been an amazing move for ST VI's story. I'm never very good at whodunnits.
I tried not to spoilerize myself with ST VI, having rummaged out every rumour on STs II-V, which was fun at the time but ruined many surprises. I bought the great "Cinefantastique" issue which focused on ST VI just before it premiered here and told myself, "Don't read the whole article yet. Save it till after the premiere. Just read the photo captions." And, of course, under the Valeris pic was a caption describing her as the traitor. Aaaaarrrrggghhhh!!!!! I'd have been safer reading the article and ignoring the captions.
So, I went into the film unsurprised, but I think I'd have guessed early on. They did drop lots of clues - but, had it been Saavik, the clues would have simply whizzed by me, I'm sure.
I think had Kirstie Alley played Saavik in each film, and had the character's dual heritage not been ignored/de-emphasised, ST VI would have been a more powerful film.