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.
All things for those who use the English language but still there's a lot to learn.
Please follow the instance's guidelines and keep on-topic. Any level of question is welcome, either from a beginner or from an almost-native speaker.
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.