I know what opinions are like.... but I'd say in general a rapidly rising fuel price will hurt Spirit (or any other LCC) harder than a major.

When fuel is low, labor is more of a factor in expense. When fuel is high, labor is a smaller factor in expense.

Throw in the ability to hedge with the larger companies (which lately hasn't helped squat), and you have kind of an ability to provide "fuel price insurance".

Of course a rapidly rising fuel price usually puts the brakes on the economy as well, so it doesn't help anyone, but in general I think the LCC's are more affected.
JMHO
As far as why NK's stock has sunk now..... no idea.