I spared her for many reasons.

Sile did nothing to personally attack Geralt. Even when he went after Sile on the tower, she said something to the effect that he should stay away because she didn't want to kill him.

That Geralt got involved at all with this kingslaying stuff was because of bad timing and a superstitious king that dragged him into a battle he didn't want to fight in. Otherwise, he could care less about politics and therefore shouldn't have any negative opinions about Sile. The Lodge was manipulating power as they've always done, albeit with a bit more regicide than usual. None of it should be Witcher business.

IMO, Geralt's primary motive was to learn about the Wild Hunt and recovery his memory. In my playthrough, he was going to stop worrying about clearing his name and just settle down with Triss until that Letho bastard kidnapped her. But then Triss mentioned he wasn't aggressive. So in the end, the only person my Geralt really wanted to kill was Szilard and everyone in the Nilfgaardian camp. Which he did.

Besides, my Geralt doesn't kill attractive sorceresses. C'mon, she's not bad... :)

The only point in letting her live is that Geralt gains in exchange a vital piece of information for himself. He learns where he must go next and he learns that a certain someone is still alive.

Beside, letting her live is the worst outcome for her. If Nilfgaard put their hands on her, she will indeed wish to be dead. Nilfgaardians hate sorcerers, especially those who don't behave. As Letho states himself, killing her is an act of mercy.

Deciding what to do with Letho was a tougher choice to me for a bunch of reasons.

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Agreed about Letho, but he did take care of Yennifer even when she made life hard for him because he felt he owed it to Geralt, and he also saved triss in the end, as well as sparing Geralt in their first encounter.

I spared her. First of all, I tried to play as much similar to the way Geralt behaved in the books as it is possible. Secondly, she provides you with some valuable information - I am sure as hell it will be included in the canon version. Finally, she and the whole Lodge of Sorceresses aren't that different than kings or Nilfgaard and some of their views (about the state of Aeddin, weakness of some kings, tolerance for inhumans and Saskia) are not utterly wrong and evil. Condemning all of mages means condemning Triss and Yennefer too and the Witcher world without them would be surely a boring place.

I killed Sile for the first time in my last playthrough. Felt so bad that I reloaded the save just to keep her alive.

There are just so many things unknown about her. Why does she wear such a strange, pointy hat all the time? What is that tattoo of? Does she like long walks on the beach?

These questions will be answered in Witcher 3, or else. Sure, she's kind of psychotic and tries to kill you with a dragon, but that kind of behavior is flirting when you do it to someone who hunts monsters. It's a classic "I only sic a dragon on you because I love you, baby," scenario.

I'm going to let her burst like an overfed tick on my next playthrough.
I thought the info I got for sparing her was worth it, but after meeting Letho again, I don't think She told Me anything more than Letho did. Maybe I'm forgetting something.
I killed Letho on my playthrough and regretted it afterwards... I think Geralt was already regretting it with the way he asks "any Vodka left in that bottle?" and then the part with the lady bug kind of finished it off nicely. I suppose depending on the way you deal with Letho, the scene with the bug could be taken as either hopeful or mournful within the context of your decisions.
But then again, I'm forgetting much about the conversation with Letho, when I spoke to him about what He'd done I'm sure He made some comment about how there wasn't really anyone around now that could finger Him and it felt like Geralt was going to be left with "King Slayer" hanging around His neck again... it felt like He had to die for Geralt to be rid of that... and that promise to Roche.

Great discussion guys and gals. I think I'm going to stick with my decision for now. Now that I think about it, killing Letho might be weighing more on my conscience that Sile.....

In terms of Letho I've killed him on both my games because I see him as still being able to be influenced by Nilfgaard, and that is dangerous. He is a Witcher, but not neutral (in the way my Roche's path Geralt feels Witchers should be), and so he needs to be taken care of. I think that the school of the viper, is just going to become a school for super soldiers for Nilfgaard, and that is not something I want to happen.

And in my Iovorth's path game even though Geralt also is aiding a particular side (Upper Aedirn and Saskia) he is doing it because he believes in the cause not because they are aiding him by rebuilding a Witcher school. Basically Geralt has convictions and Letho is all about rebuilding the school of the Viper, which he'll do anything for. He isn't a bad guy, but his larger focus is too narrow.

When I saved her, right before she teleported out, she said something lke "Alright, you've waited long enough. If you survive this ordeal, go to some city..." and I think she said where to find Yennefer.

Strangely, it didn't appear in my journal and Geralt didn't say anything about it later when talking to Letho.

Sile's totally plow-worthy. Never burn your bridges.

I played ch 3 4 times, twice for roche, and twice for iorveth , to check out the possible choices.
I went half and half on Sile, killing her twice and letting her go the the other two times, but somehow her fate never seemed too important to me on a personal level.

Letho, on the other hand, was a tough desicion. I don't know , but he felt in some ways similar to me ie Geralt , Maybe he wasn't a good guy but even after he killed Foltest ( a really awesome king ) and cut up some more heads , i couldnt really bring myself to hate him, like I hate Philipa or that Shirald Fitz .blah blah. This is one of the reasons i really like TW2, it even manages to make you feel for your enemy too.

And although i did kill him 3 out of 4 times, but that was mostly because i wanted to have a good fight with someone of my size after the dragon one rather than any desire to kill him, and most probably i'll replay the final scene before importing it to any future Expansions or TW3.

I killed him because, even though he didn't directly harm her, the guy kidnapped Triss. That's Geralt's boo, and I wasn't about to let it slide, regardless of what went down with Yennefer.

That's part of the reasons why it was difficult to me. I mean, the guy looked after Geralt woman (both of them if you choose to not rescue Triss because he's the one to do it instead). He does not show an aggressive behavior toward Geralt and have obviously no wish to consider him as an enemy and he never did anything to directly hurt Geralt and/or his friends (Foltest was a king, not Geralt's friend so he's out of this particular equation).

During the discussion with Letho I was simply experiencing a growing lack of disgust for the guy :)

She was pissing me off. Through the whole game. Guess who didn't remove the stone? >_<