I think the most important thing to remember about how the curse seals are created and why they are created different is the fact that Orochimaru probably created these seals in their different forms for his own purposes. So it probably has something to do with the cursed person's potential as a future body for Orochimaru. Maybe a heaven seal means both great power and great compatibilty, a Gaea symbol great power but less compatibility and so forth.
Please keep in mind I'm just speculating....
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