I was pushed to side with the Dalish, curing the werewolves. I wanted to side with the werewolves (even left Wynne and Alistair behind because I figured they wouldn't like it; yeah, I meta-gamed that), but I didn't have enough coercion to convince Swiftrunner and the Lady to attack the elves.
Honestly.. while I found the general Dalish plot good overall, I was a bit disappointed with the progression. I found it way too easy to get the "good" ending (gain the trust of the Dalish for the treaties, expose Zathrian's lies, and cure the werewolves). To me, it would've made more sense if the secondary option, besides going right after Witherfang, would've been to lead the werewolves to the Dalish camp in an attempt to confront and persuade Zathrian.. think about it. The Lady said the last few times Zathrian's clan was around, they tried to get him to talk, but he refused. So her plan when you showed up was.. to get Zathrian to come alone into a werewolf-infested lair and talk? What would make him any more willing to talk at your say-so than previous times? Especially since he'd be asked to come alone with you and your party (you all being outsiders, or at least not of his clan).
IMO, it would've been more sensible and dramatic if (besides killing Witherfang outright) the option was for you to lead the Lady and werewolves back to the Dalish camp (which could be explained as them having trouble tracking it down before they left the area again) to force Zathrian into discussions like they wanted. Zathrian would always refuse talking, and you'd have one last chance to side with the elves and kill the werewolves, or side with the werewolves and kill the elves.
Being coercive enough, you could open up the third option, leading Zathrian to the werewolf lair where it would play out as normal for the "good" ending. But you'd have to convince the Lady/Swiftrunner that you could talk Zathrian into coming alone and you'd side with them if things go awry (of course, you could still double-cross them and side with Zathrian when he pushes for a fight).