Lost Ark's leveling and its endgame, especially the PvP part of it, are two wildly different things. Shadowhunter has two options that change its playstyle completely, so which one is better?
Shadowhunter is a melee DPS class in Lost Ark, one that depends on filling out its gauge to be able to transform into demons, which are at the core of its design.
Once you've reached the endgame, however, you will be given the option of choosing between the Human side, with the Perfect Suppression skill, which boosts your damage in human form, with the con being your abilities having much longer cooldowns to boot. The pros of this build are the fact that its damage is very reliant, as there is no need to rely on a full uptime of your transformations, which is very skill-intensive in the endgame and can be countered straightforwardly.
The Demonic Impulse, of the Dark Side, playstyle is the complete opposite of Perfect Suppression. Here you give into your demonic nature and can bypass the 30 cooldowns between your transformations. The pros of this build, and the reason the Dark side of Shadowhunter is currently more viable, is a higher ceiling cap for damage output. This is, as previously stated, very skill-intensive though, which can be a con for some people, as the damage is not guaranteed.
The answer, then, is more complicated. If you see yourself as someone who can keep up with the needed ability rotations of the Dark Side, currently this is the most optimal build. If not, the Perfect Suppression skill can still pump out enough damage for you to find groups to play with, unlike Reaper for example.
In the end, we are not sure what kind of balance updates the developers will give out in the future, so finding a playstyle you can utilize to the maximum is still the way to go.