Actually, as it turns out, the PC version of Powerslave is different from the console versions not only in that it uses a different engine (an early version of the Build Engine), but also the levels in the PC version are completely different. The console versions use a hub system, while the PC version has linear progression. Additionally, some enemies, weapons and inventory items either work differently or are exclusive to console or PC version only.

so powerslave for console is better?

Hehe, I didn't say that :D But the SlaveDriver engine used in console versions is supposedly more advanced, as I heard (it was used for the Duke Nukem 3D Sega Saturn port, developed by the same team, Lobotomy Software). I haven't played any of the console versions myself, but from what I managed to learn, the difference is pretty huge, so I guess both the PC and console versions complement each other in a way.

I've removed incorrect info about Powerslave demos (which actually contain levels from the full version of the game (maybe with slight, hardly noticeable differences), and added info on beta versions of Duke Nukem 3D, Powerslave and Shadow Warrior.

There's a good site dedicated to the beta analysis of Duke Nukem 3D and Shadow Warrior:
http://hendricks266.duke4.net/d3dbeta.html
http://hendricks266.duke4.net/swbeta.html

The demo level of Star Wars: Dark Forces is essentially identical to the first level of the full game, save for the secret areas, some of which are different and have different goodies in them. This was done to showcase more of the full game: a careful player will find more weapons and enemy types hidden in the secret areas. To uncover some of the secrets, the player must blast cracks in rock walls using thermal detonators; this is obviously missing from the first level of the full game, as thermal detonators are not yet available on that level.

Starflight (MD/Genesis port)
Well to paraphrase a "starflt" site: I agree completely and one of my favorite games ever. Maybe because I played the port first but I can't mess with the originals for very long knowing theres this.

It looks like it was generally not unusual for console ports of various games to include certain modifications that were introduced for the sake of improvement or variety (as opposed to changes made because of console limitations like RAM or cartridge size, which usually took the form of cutting down some of the content).

Well I only ever first played F15 Strike Eagle II which was by miles better as a PC game, but Blue Sky turned Starflight into a winner for Sega. They should've ported it back over to PC.

Another one:
Star Wars TIE Fighter (interactive demo 1993)

Basically from what I read its a hybrid of TIE fighter and X-Wing. The cockpits aren't TIE fighters and the music is from X-Wing. Maybe a unique mission too?
http://www.youtube.com/watch?v=-TuOcgH0kmo

BTW thanks for the great thread I love prototypes/demos/betas etc.

I've expanded the list in the opening post, split it into categories, added more games, links and info.

Here's some:

The Lost Vikings (Sega Mega Drive)

- There were 5 new stages (one in each world) that were not present in the PC, Amiga and Super Nintendo ports, all with new end-level dialogue and respective level codes, along with a new background music track as well.

-I remember that Alien Carnage/Halloween Harry had practically two shareware versions, one under each name, and each one with a different world set of levels.
-The same tactic was used with Electro Man/Electro Body, which released two shareware versions (Polish and English I assume) with two different levels.

However, the shareware levels in both games were present in the full versions too, so I don't know if they count.