I've just played through both games, and basing it off of Petike's way of ranking the games, I've experienced it more like this:
A: Atmosphere
1. Overture
2. Tech Demo
3. Black Plague
B: Least Action
1. Tech Demo
2. Black Plague
3. Overture
C: Puzzles
1. Overture
2. Black Plague
3. Tech Demo
D: Level Design
1. Overture
2. Tech Demo
3. Black Plague
E: Story and Characters
1. Black Plague
2. Overture
3. Tech Demo
F: Gameplay
1. Black Plague
2. Overture
3. Tech Demo
G: Overall Voice Acting
1. Black Plague
2. Overture
3. Tech Demo
H: Creature Design
1. Overture
2. Black Plague
3. Tech Demo
In depth
A: To me, Overture had the best atmosphere. You felt lonely, often claustrophobic(due to tunnels), you never knew what would happen when you would finally find Red, there was more despair, and the panic effect was effective.
In Black Plague, you didn't have that feeling of loneliness, nor the claustrophobia, nor an effective panic effect - you could look directly at the enemies without it going off. Also, the enemies were mostly too human like to be creepy.
The tech demo was better, much darker and you never expected what to come. However, it felt much like a tech demo, too, and there weren't many things happening. Also pretty short. But it definetly had its points.
B: Overture could definetly have used a bit less action. I was popping propane tanks all over the place, whenever I could, and after a while, I started looking for ways to kill instead of ways to get through. All the killing gave this game too much action when in addition to the escape sequences.
Black Plague had a pretty balanced amount of action, in my eyes.
The Tech Demo had, ofcourse, the least amount of action, and that definetly was a point I liked about it.
C: Black Plague disappointed a little when it came to the puzzles. Except for the dream world puzzles, they were seldom interesting enough to make me WANT to do them.
Overture, on the other hand, had interesting puzzles. Possibly my favourite puzzle from the Penumbra series: The morse code.
The Tech Demo was short, and therefore had too few puzzles to really have a good variety, and they felt as though they were mostly there for show.
D: The level design for Overture is, in my eyes, the best, though I'd say it only surpasses Black Plague because of the first area, the mine.
Once again, the Tech Demo was too short to really have much to say, here.
E: The Tech Demo had almost no story. It had an intro, and that was about it - it sort of lost that focus once in game. No additional characters, either.
Overture lacked a bit in story - too little revealed over too much time. Overture was only scratching the surface. Red was a very interesting character, though, much more so than the ones in Black Plague, and also "Toungie", though his role was short.
Black Plague is the definite winner here. Much more was revealed in comparison to Overture. The story and characters, I would say, is what carried this game more than anything else. The characters here are interesting, especially Clarence - the way he wavers between whether he should be friend or foe is one of the really interesting things in this game. Swanson, in my opinion, could've been a bit more developed. It would also have had an a better impact on the player later.
Tuurnqait wasn't that interesting, and neither was the infected doc. (The voice com thing was funny, though.)
F: The Tech demo was like a beta version of Penumbra: A bit flawed in gameplay.
Overture's gameplay was definetly great, and Black Plague was even more polished. Automatic change from normal to interact mode and such things made my day.
G: Black Plague had really nice voice acting, and every character did a great job according to the characters they played. The same can be said for Overture, which I only put in second place because of fewer characters. Red is overall the one I think has the best voice acting, though.
The voice acting in the Tech Demo felt too soulless, and in addition, the sound quality was bad.
H: This one was actually a bit hard for me to rank, and I will explain why.
The Tech Demo didn't have many creatures but the flying thing had a really nice design, so despite being lowest on the list, I don't consider it bad.
Overture is mostly on top of the list because it introduced most of the creatures: The dogs, spiders, slugs, and rock worms. The dogs were creepy, but the spiders were mostly annoying. The rock worms were definetly scary, though. And the slugs added to the atmosphere.
Black Plague has my favourite creature design, though. The thing crawling in the walls - it's simply a touch of genious, I'd say. Certainly, it's also in Overture, though at that time you think it's one of the dogs.
I'm not going to say much about the music, except that it is all good. Personal favourite is the title theme from the Tech Demo.
Sorry about the long post, by the way, haha!