They're not that similar at all really...
Lavos is an extraterrestrial being from space which consumes the life of the planet and controls evolution in order to reach it's own ends. Once destroyed, it is dead to all timezones.
Sin is the final summoning of Yu Yevon, a weapon of war gone mad which now knows only destruction. When destroyed by the final Aeon, it merges with the soul of the person who became that final Aeon and is reborn as a new Sin.
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Magus tries to summon Lavos so that he might destroy it.
Seymour tries to become one with Sin so that he can gain eternal life and power (making him more like Queen Zeal than Magus).
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Lavos sits at the core of the planet, controlling and intellegent, soaking up the world's energy.
Sin roams the world, leaving destruction in it's wake, like a natural disaster with no apparent goal or motive for itself.
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Lavos represents the alien, the outside entity beyond our comprehension. A parasite from outside the human world that humanity must unite across generations to defeat.
Sin represents the sins of man, it is the aftermath of an apocolyptic war, which is destroyed by a new generation to bring a new age.
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Just because they both have spawn and are both big, brown and spikey on the outside and destroy things, doesn't mean they're the same.
