Which Penumbra Should I play first?

[Googolplex]

Tech Demo spoilers the whole game, the atmosphere and the core story.
The beginning is directly scary, dark environments, you get the gameplay shown.
Overture should be played first, because it begins cozy. You don't see what the game really is.
You shouldn't get a taste what Penumbra awaits to you.

Overture is the true beginning, so play it first.