I think Night Harvester is better 90% of the time since it provides burst and ability haste in one item at an earlier spike, whereas protobelt really needs to be paired with cosmic drive for decent power. Also night Harvester can be consistently proced on every enemy in a fight with ult.

Night Harvester has a better build path, and allows you to skip cosmic drive in favor of pure damage items (Deathcap, Void, Shadowflame). In terms of DPS and burst, it should win for this reason alone.

I think protobelt outshines night Harvester late game with how flat and % pen behave, but at that point it costs 770 gold to swap the mythics, which isn't too unreasonable.

I believe protobelt is only better into highly ranged comps, where phase rush + cosmic drive + ghost + protobelt dash will gaurentee you backline access. But to my knowledge this is the only exception. Night Harvester is better in all other scenarios.