After a lot of playing all three I changed my mind.
Synth ascension is good. You'll need to plan ahead with it, as you don't upgrade your bonuses (like droids, who can be specialists with no CG upkeep and very little housing) but throw away your previous bonuses and get totally different set.
- the main advantage is insane robot assembly speed
- synths have 20% production without traits - 10% from being synths, 10% from synth ascension
- some of machine traits are quite nice - like +10% happiness
- up to 70% upkeep reduction (20% F.materialist, 10% civic, 5% mechanist civic, 10% trait, 10% flesh is weak, 10% edict, 5% event edict). Live in utopia without any cost! Well, I prefer different civics really, but even 20-40% reduction make synth specialist or ruler cheaper then any organic.
- no penalty in assembly speed if you select the model, unlike bio (like using +energy model on generator worlds)
- be immortal without spending trait points for that (bio can be more reliable to live long, but it cost 4-8 points just to stay alive)
And you still can use organics if you wish even more combined growth speed - you'll get much more then bio who also build synths.