I'm in the minority, but I would not have liked Saavik. I can appreciate that David's death in TSFS would have colored her outlook on the Klingons, as Kirk's was likewise. But the notion that this would drive her to conspire to perpetuate continual war rather than accept peace is incredibly illogical and dangerous. I think having Valeris as a separate character works better because she symbolizes how many "average" Starfleet officers would have felt. She works for me because she isn't a character that's been carried over through several movies.
I've also heard the story that Kim didn't want to simply be the new Saavik, but wanted a distinct character.
I disagree. For one, if Saavik had been the conspirator it doesn't mean she supported continual war. Admiral Cartwright and the hardliners thought that the Praxis situation opened up the opportunity for a final or at least decisive war against the Klingons, to break them once and for all. The end of TUC left the Klingons as tenuous allies, but still very strong, and we see even in the TNG and DS9 time frames that the Klingons were not completely committed allies with the Federation nearly a century after TUC, and in DS9 they eventually went to war again with the Federation.
If it had been Saavik at least the audience would've had something to latch onto in an emotional and visceral way to understand Saavik's actions. We don't know why Valeris did what she did, outside of her assertion that the Klingons were too dangerous and couldn't be trusted. So we still got what could be a considered illogical and dangerous position from Valeris, though without an explanation for why she arrived at that point in the film. Personally I don't think it was illogical. It was dangerous, but the peace deal Spock was proposing also was dangerous too.
Valeris was looking at the nearly a century of Klingon aggression and it made more sense not to trust them than to trust them. How was the Federation to know or truly trust Gorkon's intentions in light of the decades of history that told them otherwise. I don't think the conspirators went about it the right way but I don't think their suspicions were wrong on their face. Unfortunately the side effect of all that suspicion did become unreasoning bias and hatred against the Klingons and I don't agree with that. But the initial wariness I do agree with. And even with the bias that was exhibited by various Starfleet officers, it was regrettable, but also made sense due to the long history of hostility between the Federation and the Klingons. If the Starfleet officers didn't harbor ill thoughts about the Klingons then that would've been unbelievable to me.
Valeris wasn't an average Starfleet officer. She was one of the best in the Academy and Spock's heir apparent. The two crewman that carried out the assassination against Gorkon were Average Joes who were easily dispatched by Valeris. Valeris was higher up than they were, but not as high as Admiral Cartwright.
I would be fine either way with Valeris or Saavik in TUC. I liked Valeris. I liked Saavik. I do feel though that including Saavik would've been more dramatically powerful and emotionally affecting, seeing her argue with Kirk over David's memory and perhaps even the need to avenge him, or Spock struggling to reconcile his pride and disappointment with her, and the mind meld scene would've been even more gripping and terrible because we 'knew' Saavik in a way we didn't know Valeris.
I also think it sucks that they didn't use Saavik because they never revisited the character. I read that she was supposed to be on TNG once, but that didn't happen. It would've been great to see her on TNG or DS9 or something.