Slaves vs robots

[Mr. Wiggles]

Slaves seem a no brainer, especially if you choose the biological ascension path, but synth servants with the right trait occupy almost half housing space compared to slaves, thats the main advantage.

[Azor]

you can have synth slaves too

[Nightmyre]

You can also have both biological slaves and robotic workers.

[Locklave]

Slave can't be a late game crisis, robots can. You need to specialize which ever one you go with.

Slaves are cheaper to maintain and there are a number of ways to improve their production.

Also servant I think that's the status, is a specialized type of slave that requires nearly nothing and gives basically free amenities. If you found charismatic aliens they'd be perfect for that role.

[Malaficus Shaikan]

In my experiance's.
Slave have better minning value, beter farming value and dont require to be modified to become servants just a swich into policy into domestic servants.

That said if you go full synth accesion technocracy mechanist build.
You robots will quickly surpass slave's the moment you unlock droids.
Robots can colonize all worlds, dont have a negative impact on public order(meaning less slots wasted on uprising supression) and when you go full synth you can have an empire that has all the benifit's of a machine empire and normal empire.

So unless you go full synth accesion fanatic materalist technocracy mechanist slave will be beter for 90% of the game.
That said there are a few downside's to only slave's.
1: ai will hate you.(and in multiplayer it will put you on my to conquer list)
2: Slave's cause unrest and while it is easy to keep the unrest down it still require a wasted building slot and if you happen to have a bit of a crisis preventing you from keeping order on planet slave uprising can get out of hand quickly.
Still you need to be losing badly for slave uprising to be a problem.

In short both are good depending on how you plan to play.
I would suggest slave's guilds for the simple fact that it will automate your slavery on your worlds and avoid you needing to manual keep slave numbers on world form going balistic.
It also allows you to start with slave's and that is a major early game income increase.

So slave's vs robots depends on a very simple question:
Do i consider the hassle of supressing slave's worth it?

[Mr. Wiggles]

In all sincerity I never ever had any problem with my slaves happiness, I try to keep them happy until I unlock the advanced species traits, then I nerve staple all of them.
I use three slaves model to keep things simple and micro at bay: one specie optimized for the chattel labour, one optimized to be a good servant (nerve stapled, trade value and amenities boost) and one specie optimized to be eaten (nerve stapled, delicious, fertile).

I let the slaves grow and migrate until they reach a good pop percentage, then I set pop and migration controls and micro them a bit (nothing special, I just remove the chattels from my arcologies and tech worlds). Repeat when your citizens outnumber the slaves again and you need raw resources.

For the specie used as livestock I activate the migration controls and load them on a few planets optimized as agri worlds and thats it, literally more food than what you will never need.

Slaves+ascension path are superior to synth servants in terms of raw resources output, one advantage I think synths have is they can be employed in specialist jobs even when the AI right is set to servitude, bit i am not sire of that, I dont bother to have robots and risk a machine uprising.