Which model do you think is best in terms of difficulty of a game?

Liner: Difficulty increases at a constant amount so you will have to be able to get stronger at a good rate.

Exponential: Difficulty gradually increases in the beginning so you can get the hang of things, then mid-way through the game starts becoming a challenge and forces you to try harder in order to keep playing.

Step: Difficulty jumps every once in awhile and stays like that until it starts becoming easy then jumps again.