On. I like the natural progression of the game as far as the life cycle goes. Besides, I really enjoy watching the genetics change with each generation of my favorite families. Lately, I've been rotating through the premades and progressing them as well because I really do want to see the next generation of families like the Goths, Pancakes, Landgraabs, Scotts, Lees, etc...
I used to play on short lifespan before we had so much content to play with. Now I play on normal or long depending on the family, the save, and how much I like and want to keep them around.