Okay, time to respond to some earlier posts that I never got around to...
How is the cycle broken? There was peace between quarians and geth before the uprising, after all.
Sure there was, if you count servitude as equivalent to peace. Which... I don't know, do you treat servitude as equivalent to peace, not-secretly-Admiral-Xen? 
As for both sides choosing it, I would argue that peace was in a way forced on the quarians, since part of if not the primary reason Han'Gerrel relents from attacking is the persuasive power of space jesus/ Shepard convincing him that he won't win against the Reaperized geth. Obviously, there are differing opinions among the quarians since they are organics, and thus individuals, but all it takes is a dedicated minority to reignite the conflict.
This argument goes for the other side too. Remeber when a small minority of geth decided that the Reapers were gods that and all meatbags must die? Remeber how they were narrowly stopped from forcibly rewriting the entire geth consensus into accepting this ideology? What's to stop something similar from happening?
I treat the Rannoch peace as possibly-unique. I feel that it's worth letting it play out for a while before declaring that it'll never work, and that it has the potential to defy the cycle. Merely the potential, nothing more. Or maybe I'm just a raging optimist.
Regarding both sides choosing the peace: The Geth didn't want to fight the Quarians, and they didn't start the Returning-War. Meanwhile, the civilian fleet, we're told (during Koris' mission), also didn't want this war. Clearly Han'Gerrel, Xen and a substantial proportion of the Quarian military did want the War, but the civilian majority on one side and the entire Consensus on the other side both didn't want to fight.
You're right. A dedicated minority probably could reignite the conflict. But why would they?
The Quarians have their planet back, and they remember what happened the last time that they got on the Geth's bad side. I don't feel that many Quarians would be willing to risk losing Rannoch again when they've only just got it back. Besides, they've got an opportunity to rebuild their lives rather than focusing on old grudges.
Meanwhile the Geth aren't slaves anymore. Heck, they don't even need Rannoch. The Geth no longer have their backs against the wall, their existence is no longer threatened, and they're at peace with their creators and the rest of the galaxy. I don't see them willing to risk any of that. If they want to build a Dyson Sphere again, no one is likely to interrupt them this time.
As for the Heretics - their motivation for attacking organics was as part of a deal - the Reapers would give them tech, meanwhile they'd help the Reapers with the harvest. But, with the possible exception of Renegade Control, that's not really an option anymore, is it?
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Finally: Why do I consider Rannoch-peace as possibly unique? Besides the fact that it's so hard to achieve?
When the Catalyst explains the cycle, it refers to how Synthetics are created to serve, and that when they grow beyond their creators, they naturally rebel against their Creators to escape that servitude. With Rannoch, we've already had that chain of events - and for once, the Creators survived the rebellion and were able to re-establish peace on a more equitable level. There's no indication in game that anything like that has ever happened before (except for maybe the Zha'til, but the Reapers hijacked that peace before it could play out).
Maybe it has happened before, and maybe there are other reasons why the cycle would re-assert itself after this point. Or maybe it hasn't happened before, and picking Destroy blows up the one good chance we've ever had for long-lasting Synthetic-Organic peace.