Similarities:
Both shows are romance fantasy/xianxia with aquatic demons (slightly different takes on it) and include the tension & cooperation between different ‘races/tribes’

both have slight love-triangles but the SMLs are mostly silently pining.
ML & SML have a grudging cooperation and evolving friendship, not just over shared love interest but also due to being leaders.

In both couples they are trying to protect their partners and instead are ready to sacrifice themselves
(this includes hiding things from their partners, I just mention it because I know some viewers are sensitive to this and don’t want to watch shows like that, but it’s not unusual for self-sacrificial xianxia plots ^^)


differences:
besides storyline differences, Mirror: a Tale of Twin Cities has slower storytelling and the couple is more on a ‘grown-up & introverted’ vibe while Song Of The Moon main couple is more ‘extroverted & youthful’

Each drama has their weak points, but I still overall enjoyed both and personally appreciate that the ‘love-triangles’ are more handled as ‘grudging & a bit awkward friendships’ between MLs & SMLs :)