Why choose Sejuani over Amumu ?

I already own Amumu, and I love him for his fairly simple yet extremely strong initiation, so naturally I am curious to whether I should buy Sejuani. However at the moment I can't really think of any good reason why Sejuani would be picked over Amumu, I feel like Amumu just fits into team comps better with his more reliable ult. Sejuani mains halp.
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