Lingering on the horizon in my experience is used to convey optimism about future potential opportunities. It’s out of reach but just around the corner.

Looming ahead sounds more like an impending negative consequence.

My 2 cents anyway.

If you used either, I’d understand perfectly what you meant.

EDIT: I should point out that “looming” suggests something ominous or negative. Where “lingering” doesn’t necessarily mean a negative upcoming event, only one that is likely unavoidable.