Thanks, I've had a look at the games you all have mentioned.
To Pappenheimer: You're right but I know the differences and the gameplay determines the level design, that is why I gave two examples above.

I don't want to open a new thread for the following question, so here we go:
What do you guys think is an acceptable time of playing for such a (low budget) game?
Please consider the time it takes to finish the game the first time and to complete it by 100%.