Dark Sight is better if it has no setting-dependent implications you want to avoid
These two Traits being side-by-side without any obvious reason to pick the lesser one is a side effect of 1) Mythras being a descendant of games set in Glorantha, and intended to allow Glorantha play, and 2) Mythras being a “kit” system of modular rules for the GM to choose from to realise their setting, rather than one with a fully realised integration between rules and setting.
(2) is relevant because Mystics can Invoke Traits based on what fellow beings exist in the setting:
Traits are specialised physical, mental and emotional endeavours akin to the those found for many creatures.
The GM is within their worldbuilding authority to not include specific Traits in a given setting, and possibly also say that they are therefore not available to the Mystic. Perhaps Dark Sight is not a thing in a given setting, and a Mystic can't emulate it.
Or perhaps it isn't a naturally-occurring thing in the setting, but it's still something Mystics can do that nobody else can — that's a viable bit of GM worldbuilding. In such cases, yes, Night Sight is redundant.
But what about those implications I suggested? Well…
(1) is relevant because Glorantha is where Dark Sight comes from: Trolls have a “dark sight”, which isn't really “sight” as we know it (hence the scare quotes around “see” in the Trait). Due to Gloranthan trolls’ inherent connection to the Darkness rune (and everything real/mythical that implies), they don't experience darkness as surface races do. It's just not an impediment to their senses — darkness is their natural mythical medium of interaction with the world, including sensing it.
A Mystic choosing to Invoke the Trait Dark Sight, in a setting where Dark Sight is inherently connected to Darkness, might have reasons to avoid embodying Darkness. Or maybe it's fine. But if a given setting ties Dark Sight to the powers of Darkness and a Mystic has a reason to avoid associating with the powers of Darkness, they might be happier/safer Invoking the Night Sight of nocturnal animals that is connected to (for example, perhaps) the Beast powers instead of to elemental Darkness.
If none of that applies though? Yes, Dark Sight is strictly better than Night Sight. If you're not playing in Glorantha or a similar setting where what powers you associate with matter socially and politically and can make the difference between life and death, if you're instead in a setting where they're just two morally and politically neutral options for your Mystic to choose from, then Dark Sight is superior in all ways.
So check with your GM!