I learned Nightshade the other day, then decided to pull in Excel and document all my servitors stats, crystal usage, shot usage, skill, and downsides if any. The summary is as follows, at level 56 and all servitors levelled up:

Shadow: Slowest speed, passive (and undocumented) VR, highest crystal usage but otherwise Patk only about 88 lower than soulless. An absolute boss for 2-3x hp mobs, only really needs heals on 2 or more mobs at once, or mobs in the yellow/light red range.  Since he’s so slow, you can easily run along side him and melee if you so desire, which is what I do.  Takes 30% exp though but well worth it.

Soulless:  Fastest speed, highest patk, two skills (poison and corpse explosion, neither really worth doing imo), only takes 10% experience BUT has a MP draining debuff they place on the summoner that brings your mana regen to a standstill.  Should stick to 1x hp mobs that are white or lower, or you’ll be doing a lot of healing that will slowly drain you to nothing thanks to the debuff. May as well just stand around and let him attack though because he’s quicker and the mana drain is a bitch.

Nightshade: Second fastest, only takes 5% exp but has patk far lower than the rest.  Think like 500-600 less.  Has a taunt+curse combo that includes reducing pdef and mdef both single target and AOE version.  However has ZERO mitigation skills to go with those taunts and unbelievably his pdef is the SAME AS THE OTHER PETS. He has a big heal but it’s a 1 to 1 health transfer from the servitor to the selected target.  Could be used as an off-healer though if like, all the other healers were afk or oom or something. No idea why you’d have this guy out at all though. As of right now I have no clue what niche he fills.

Silhouette: Slow as Shadow for some reason, and takes a whopping 90% exp.  Only takes 1 D crystal to summon though.  Has the same patk as shadow and a Steal Blood attack to attack the enemy and heal the servitor. Likely just for pvp.

With all that being said, I will use Shadow for yellow mobs, 2-3x hp mobs, or when I need to be pretty safe.  He’s a fucking BOSS.

Soulless is quick and strong if you are in a safe scenario with 1xhp mobs that won’t gang up on you, white or green is perfect. Also takes considerably less exp.  You really need herb drops to keep his hp up because your mp won’t keep up otherwise.

The other 2 are not worth using as far as I can tell at the moment.