These are framing args.
Link determines uniqueness means even if there's some chance the aff is ahead on the uniqueness debate, the plan would be so large of a break from the status quo that it locks in whatever change the DA is about.
Uniqueness determines the link means that because the status quo is a certain way, links are only true in the direction away from it.
So with midterms , neg says dems winning now, aff flips it for a GOP win. The aff would say GOP win now.
So the neg, making a link determines uniqueness arg, would say that the aff locks in a GOP win, so the neg only needs to win that it's possible dems could win.
The aff saying uniqueness determines the link would say that because the GOP will win now, it doesn't matter if the aff makes it an even more overwhelming GOP victory.
The reason link determines uniqueness makes more sense is because it is a tiebreaker argument. To win uniqueness determines the link, the aff has to almost entirely win the uniqueness debate without a doubt. If the neg is getting that beaten on the uniqueness debate, of course the link doesn't matter.