Cloud Sync is vital
Whatever you decide, I think it's absolutely vital that you get a solution that allows a completely frictionless synchronization between your desktop and your iPhone. In other words, it has to feel like you only have "one list" that you can look at from either place, rather than two lists that you have to worry about keeping up to date.
As a practical matter, I think that rules out Things (at least for me, at least for now).
I tried Things and Toodledo, but eventually settled on OmniFocus, and I'm happy with it. It's a clean interface and a well-designed set of applications.
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